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NONLINEAR OPTICAL STUDIES OF
POLYDIACETYLENES

By

Selezion Afeworki Hambir

A DISSERTATION

Submitted to
Michigan State University
in partial fulfillment of the requirements
for the degree of

DOCTOR OF PHILOSOPHY

Department of Chemistry

1995

ABSTRACT

NONLINEAR OPTICAL STUDIES OF POLYDIACETYLEN ES
by

Selezion Afeworki Hambir

The main objective of this dissertation is the investigation of conjugated polymers as
potential photonic switching materials. Present day information transfer is done by means
of electronic signal processing. Faster information transfer will require replacement of
electrons with photons as information carrying units. Such an application requires
eflicient switching devices and an efficient switching techniques. Polydiacetylenes (PDAs)
are promising candidate photonic switching materials because of their large third order
nonlinear optical response. Before these materials can be incorporated into optical
switching devices, their energy relaxation pathways and ability to be processed into

devices has to be investigated.

This dissertation is composed of two parts. The first part (chapters 2 & 3) were conducted
to evaluate the feasibility of storing energy in a polydiacetylene with carbazole side
groups, FDA-DCHD. Polydiacetylene DCHD was studied both at 300K and 10K and we
found that energy deposited on the side groups migrates rapidly (<10ps) to the PDA
backbone. Relaxation of this excitation has the same spectral and temporal signatures as
relaxation of the backbone excitonic resonance. Comparison of these data to the
electroabsorption response of DCHD confirms excitation transfer rather than charge

transfer. This energy transfer is consistent with a Forster through-space mechanism. At

low temperature the slow population recovery signal seen subsequent to excitation
transfer to the polymer backbone shows an excitation energy dependence that is indicative
of the existence of different conformers under the excitonic envelope. Thus, long term

optical energy storage in PDAs does not appear to be feasible.

The second part of this thesis (chapters 4 & 5) was conducted to investigate systematically
how the nonlinear optical response of the polydiacetylene poly(4BCMU) varies as its
morphology is adjusted fi'om crystalline to highly disordered materials. A pump-probe
method of accessing x“) which makes use of the large linear absorption of the conjugated
polymers was developed. This nondegenerate wave mixing technique (inverse Raman
spectroscopy) is very sensitive to subtle chemical structure and thus provides opportunity
to investigate a novel switching scheme by altering polymer morphology. The magnitude

(3) is related to the oscillator strength of the exciton that dominates the absorption

of x
edge in PDAs and our pump-probe measurement scheme couples vibrational modes to the
exciton to enhance the nonlinear optical response. The dynamic Stark effect has been
measured both in the crystalline form of poly(4BCMU) and in films possessing varying
degrees of disorder. The macroscopic alignment of the polymer chain in these films is
controlled by stretch orientation. After characterizing the linear optical properties of these
materials, the coupling between the vibrational and electronic structure of these systems
was measured using stimulated inverse Raman Scattering. In PDAs disorder can be used
to enhance the nonlinear response by a factor of ~2 per chromophore unit. Stretch

orientation of disordered films aligns individual chromophores and thereby a nonlinear

response comparable to that of the highly ordered crystalline polymer can be achieved.

Acknowledgments

I would like to thank my advisor, Dr. Gary J. Blanchard for his valuable advice, patience
and time he took to walk me through the tough experiments and most of all for
maintaining fiiend-fiiend type of relationship during my graduate career. I would also like
to extend my sincere gratitude to my committee members Dr. G. T. Babcoclg Dr. J. S.
Ledford and G. L. Baker. Special thanks goes to Dr. G. L. Baker for providing me with all
the polymers I needed for this project. My thanks also go to the members of the Blanchard
group, past and present. I really enjoyed working with you, and wish all the best of luck in
what you do. I wish to thank the people in the machine shop and electronic shop for their
efficient and prompt reply to my needs. My special thanks go to Tom Carter for helping
me with computer whenever I am stuck. Finally, I would like to thank my brothers, sisters

and friends for their understanding, support and love.

iv

To:

Eritrean martyrs.

TABLE OF CONTENTS

LIST OF TABLES

LIST OF FIGURES

INTRODUCTION

Chapter 1. SYNCHRONOUS PUMPING OF TWO DYE
LASERS USING A SINGLE UV EXCITATION

SOURCE

1 .1. Introduction
1.2. Experimental

1.2.1. Source laser

1.2.2. Dye lasers

1.2.3. Time domain measurements
1.2.4. Frequency domain measurements
1.3. Results and Discussion

1.4. Conclusions
1.5. Literature Cited

Chapter 2. EXCITATION MIGRATION IN THE
POLYDIACETYLENE DCHD

2.1 . Introduction
.2. Experimental

2.3. Results and Discussion

2.4. Conclusion
2.5. Literature Cited

vi

ix

10
11
11
12
12
13
14
22
23

25

26
29
31
42
43

Chapter 3. LOW TEMPERATURE TRANSIENT OPTICAL
SPECTROSCOPY OF POLYDIACETYLENE
DCHD. EVIDENCE FOR A DISTRIBUTION

OF SIDE GROUP ORIENTATIONS 45
3. 1 . Introduction 46
3 .2. Experimental 47

3.2.1. Materials and temperature control 47

3.2.2. Pump-probe spectroscopy 48
3.3. Results and Discussion 50
3.4. Conclusion 61
3.5. Literature Cited 62

Chapter 4. DISORDER INDUCED ENHANCEMENT OF
THE THIRD ORDER OPTICAL NONLINEARITY

IN A CONJUGATED POLYMER 64
4.1. Introduction 65
4.2. Theory 69
4.3. Experimental 73

4.3.1. Laser spectroscopy 73

4.3.2. Material processing 75
4.4. Results and Discussion 76
4.5. Conclusion 90
4.6. Literature Cited 91

Chapter 5. THE EFFECT OF STRETCH ORIENTATION
ON THE LINEAR AND NONLINEAR OPTICAL

RESPONSE OF POLY(4-BCMU) FILMS 93
5.1 . Introduction 94
5 .2. Experimental 95
5.2.1. Nonlinear laser spectroscopy 95
5.2.2. Linear spectroscopy 96
5.2.3. Material processing 96
5.2.4. Substrate 97
5.3. Results and Discussion 98
5.4. Conclusion 125

5.5. Literature Cited 126

vii

Chapter 6. FEASIBILITY STUDY OF DEGENERATE
FOUR WAVE MIXING

6.1 . Introduction
6.2. Experimental
6.3. Discussion
6.3.1. Unit Conversions
6.3.2. Magnitude of the Expected Signal
6.3.3. Expected Signal to Noise Ratio
6.4. Literature Cited

Chapter 7. SUGGESTED FUTURE STUDIES
Literature Cited

viii

129

130
132

134
135
137
139

140
142

Table 1.1.

Table 1.2.

Table 4.1.

Table 4.2.

Table 5.1.

Table 5.2.

Table 5.3.

Table 6.1.

Table 6.2.

LIST OF TABLES

Output parameters of stilbene and coumarin dye lasers modeled
by the noise burst model.

Cross correlation widths for different dye laser wavelength
combinations. Wavelengths for the two lasers are indicated

on top and side. Cross correlation time widths are from fits

of the data to a Gaussian function.

Experimental v-X transition cross-sections and maximum AT/T
values for backbone double and triple bond stretching resonances
in crystalline and spin cast films of poly(4BCMU).

Experimental frequencies of C=C and CEC stretching resonances
for spin cast film poly(4BCMU) as a function of excitation energy.

Cross sections for ground state vibration-to-excited electronic state
transitions in poly(4BCMU).

Comparison of elongation ratio and optical dichroism at 2.29 eV.

Excitation energy dependence of vibrational resonance frequencies
of spin-cast and aligned poly(4BCMU) films.

4 4
Conversion table between 1532, [cm—2) and 15931)[—C—:1] .
sC J

Average and peak E values for E = 10'7m and different x“) values.

ix

17

20

87

88

119

121

124

135

137

Figure 1.1.

Figure 1.2.

Figure 1.3.

Figure 1.4.

Figure 2.1.

Figure 2.2.

Figure 2.3.

Figure 2.4.

LIST OF FIGURES

Gain curves for five laser dyes spanning the wavelength range
between 405 nm and 555 nm. Conversion efficiency is defined
here as the ratio of the input average power to the output
average power.

(a) Frequency spectrum for a stilbene 420 dye laser operating at

440nm presented with a best fit Gaussian function for the spectrum.

(b) frequency spectrum for a coumarin 460 dye laser operating at
485 nm, presented with a best fit Gaussian function.

(a) Autocorrelation trace corresponding to the frequency profile
shown in Figure 2a, presented with a fit to Equation 2. Parameters
of the fits are presented in Table I. (b) Autocorrelation trace
corresponding to the frequency profile shown in Figure 2b,
presented with a fit to Equation 2.

Unidirectional cross correlation for the two pulses detailed in
Figures 2 and 3 together with a fit of the data to symmetric
Gaussian function.

Structure of polydiacetylene DCHD. The long axis of the carbazole
side group is tilted 81° and the short axis 43.3 ° with respect to
polymer backbone in the crystal. The arrows indicate the transition
dipole moment directions.

Linear absorption spectrum of a ~16OOA thick crystal of DCHD
in the region of the polymer backbone excitonic transition.

Transient response of DCHD for excitation at 1.908 eV, direct
excitation of the exciton. The fast response (0 ps, I) is scaled to
the linear absorption spectrum. Slow responses are shown for
30 ps (0) and 125 ps (A) delay times.

Transient fast (0 ps, I) and slow (30 ps, 0; 125 ps, A) responses
of the DCHD excitonic transitions for excitation at 4.13 eV, the
long axis polarized transition localized on the carbazole side group.

15

16

18

21

27

30

33

34

Figure 2.5.

Figure 2.6.

Figure 2.7.

Transient fast (0 ps, I) and slow (30 ps, O; 125 ps, A) responses
of the DCHD excitonic transition for excitation at 3.57 eV, the short
axis polarized transition localized on the carbazole side group. 35

Time-scan of the pump-probe signal for excitation at 1.908 eV and
probing at 1.879 eV. The magnitude of the slow absorptive signal
decays exponentially with a time constant of 3.3 i 0.3 ns. 37

Comparison of the slow response reported here (solid line with

data points) with the electroabsorption response of DCHD, taken
from reference 16 (dotted line). Signal intensity scales are arbitrary. 40

Figure 3.1.

Figure 3.2.

Figure 3.3.

Figure 3.4.

Figure 3.5.

Figure 4.1.

Figure 4.2.
Figure 4.3.

Figure 4.4.

Linear absorption spectrum obtained from Kramers-Kronig
transformation of the reflection spectrum of a thick crystal of
DCHD (z IOOum) in the region of the polymer backbone excitonic

transition at (a) 300 K and (b) 10 K. 49
Time scan of the pump-probe signal for excitation at 1.937 eV
and probing at 1.925 eV. 53

Transient response of DCHD for excitation at 1.93 7eV direct
excitation of the exciton transition. Fast response (0 ps, I) and
slow response (30 ps, 0 ; 125 ps, A) are shown. 54

Transient responses of the DCIH) excitonic transition for excitation

at 3.57eV, the short axis polarized on the carbazole side group.

Fast response (0 ps, I) and slow responses (30 ps, 0; 125 ps, A)

are shown. 55

Transient responses of the DCHD excitonic transition for excitation

at 4.13eV, the long axis polarized on the carbazole side group.

Fast response (0 ps, I) and slow responses (30 ps, 0; 125 ps, A)

are shown. 56
Poly[5, 7-dodecadiyne-1, 12-diolsbis(n-butoxycarbonymethylurethane]

or Poly(4BCMU). The dashed lines indicates hydrogen bonding

between the side groups. 67
Schematic three level system. 70.

Nonlinear susceptibility as a function of potential well displacement. 74

Linear absorption spectra of crystalline and spin cast film forms
of poly(4BCMU). For the crystalline polymer, to... = 1.96 eV

xi

Figure 4.5.

Figure 4.6.

Figure 4.7.

Figure 4.8.

Figure 5.1.

Figure 5.2.

Figure 5.3.

Figure 5.4.

Figure 5.5.

Figure 5.6.

and for the spin cast film, (1).,x = 2.29 eV.

Experimental data points and calculated fit (solid line) for
a) stimulated inverse Raman spectrum of crystalline poly(4BCMU)
for top = 1.774eV and b) for co,, = 1.741eV.

Experimental data points and calculated fit (solid line) for
a) stimulated inverse Raman spectrum of crystalline poly(4BCMU)
for to, = 1.701eV and b) for top = 1.653eV.

Experimental data points and calculated fit (solid line) for
a) stimulated inverse Raman spectrum of spin cast poly(4BCMU)
for 0),, = 2.141eV and b) for 0),, = 2.113eV.

Experimental data points and calculated fit (solid line) for
a) stimulated inverse Raman spectrum of spin cast poly(4BCMU)
for (1),, = 2.083eV and b) for (up = 1.978eV.

Linear absorption of a poly(4BCMU) film cast on FEP for
elongation ratio: (a) [/80 = 5 (b) 8/20 = 1.

Linear absorption of (a) crystalline poly(4BCMU) (b) freshly
prepared spin cast poly(4BCMU) on a quartz substrate and

(c) elongation to (Mo = 5 for a poly(4BCMU) film cast on FEP
copolymer substrate.

Experimental data points and calculated fit (solid line) for
a) stimulated inverse Raman spectrum of spin cast poly(4BCMU)
for 0),, = 2.141eV and b) for top = 2.113eV.

Experimental data points and calculated fit (solid line) for
a) stimulated inverse Raman spectrum of spin cast poly(4BCMU)
for top = 2.083eV and b) for top = 1.978eV.

Experimental data (points) and calculated fit (solid line) of the

stimulated inverse Raman spectrum of a spin cast film poly(4BCMU)
on a FEP copolymer substrate, 0130:], for (a) mm = 2.142 eV and

(b) mm= 2.109 eV.

Experimental data (points) and calculated fit (solid line) of the

stimulated inverse Raman spectrum of a spin cast film poly(4BCMU)
on a FEP copolymer substrate, €/€o=l, for (a) topump = 2.077 eV and

(b) wm= 1.978 eV.

xii

77

80

81

82

83

101

103

104

105

107

108

Figure 5.7. Experimental data (points) and calculated fit (solid line) of the
stimulated inverse Raman spectrum of a spin cast film poly(4BCMU)
on a FEP copolymer substrate, €/£’°=1.5, for (a) mm = 2.142 eV
and (b) mm= 2.109 eV. 109

Figure 5.8. Experimental data (points) and calculated fit (solid line) of the
stimulated inverse Raman spectrum of a spin cast film
poly(4BCMU) on a FEP copolymer substrate, £’/€O=1 .5, for (a)
mm= 2.077 eV and (b) mm= 1.978 eV. 110

Figure 5.9. Experimental data (points) and calculated fit (solid line) of the
stimulated inverse Raman spectrum of a spin cast film
poly(4BCMU) on a FEP copolymer substrate, 020:3, for
(a) 03pm: 2.142 eV and (b) (0pm,: 2.109 eV. 111

Figure 5.10. Experimental data (points) and calculated fit (solid line) of the
stimulated inverse Raman spectrum of a spin cast film poly(4BCMU)
on a FEP copolymer substrate, M753, for (a) cop...“p = 2.077 eV and
(b) mm: 1.978 eV. 112

Figure 5.11. Experimental data (points) and calculated fit (solid line) of the
stimulated inverse Raman spectrum of a spin cast film poly(4BCMU)
on a FEP copolymer substrate, €/€o=5, for (a) com= 2.142 eV and
(b)co,,.....,=2.109 eV. 113

Figure 5.12. Experimental data (points) and calculated fit (solid line) of the
stimulated inverse Raman spectrum of a spin cast film poly(4BCMU)
on a FEP copolymer substrate, iii/30:5, for (a) com: 2.077 eV and
(b) mm=l.978 eV. 114

Figure 5.13. Experimental data (points) and calculated fit (solid line) of the
stimulated inverse Raman spectrum of a freshly prepared spin cast
poly(4BCMU) on quartz substrate for mm = 1.978 eV including
in the calculation (a) two excitonic resonances ((1),. = 2.02, 2.29 eV)

(b) single excitonic resonance ((1)" = 2.29 eV). 117

Figure 5.14. AOL, the nonlinear response, normalized for sample thickness and
laser intensities for the various forms of poly(4BCMU) as a function

of stretching ratio for different excitation frequencies. 123

Figure 6.1. Schematics of the optical beam geometry in the plane of the sample
for the folded boxcars geometry. 131

Figure 6.2. DFWM schematic energy level for the experiments. 133

xiii

Introduction

It is expected that the next century will see a wide applications of optical technology for
transmitting, receiving and processing information. At present, the well established
technology is one in which electrical signal are converted into pulses of light by rapidly
turning semiconductors laser on and off. The resultant optical signal is then sent down
optical fibers and received at the other end by a solid-state detector. The optical signal
carried on the optical fiber is converted to an electrical signal for manipulation and
decoding. Subsequent switching and signal processing is, by and large, performed by
conventional electronic means. Therefore, in today’s information transfer mechanism,

optical signal finds use only in the transmission of information through fiber optics.

For faster information transfer one obvious thing to do is to replace electronic devices
with their optical analogs. This would provide a direct means of switching optical signal
without conversion back to electronic signal. An all-optical switch, which processes light
by light, would circumvent this conversion complexity, without sacrificing the speed of
light transport by the signal. An optical switch must function macroscopically much like a
mechanical switch. The setting corresponding to “on” must allow light to pass through

while “ofl” blocks transmission.

2

Photonics are expected to be superior to electronics for the following reasons: 1) a
photonic switching cycle can, in principle, take place within femtoseconds, thus providing
a gain of speed over that of the electronic process; 2) working at optical frequencies
provides tremendous gain in the bandwidth of the information processing devices; 3)
optical processing functions are generally free fi'om interference by electrical or magnetic
sources; 4) freedom from cross-talk i. e. two beams of photons can pass through one
another with negligible interaction. The same is obviously not true for electrons flowing in
crossed wiring. It is the combination of massive parallelism, and cross-talk and

interference-free interconnection, that gives optics its greatest advantage over electronics.

The most critical need in this field today involves finding materials to carry out the active
switching fimctions of optical devices. These materials will be used to construct photonic
devices and are to photonics what semiconductor materials are to electronics. Optical
switches and amplifiers will perform the fitnctions now performed by transistors in
electronic devices. All-optical devices rely on nonlinear optical materials. A nonlinear
optical material may be defined as any material that changes any of its optical properties as
result of illumination. These changes can be quantified by observing the behavior of the
optical constants upon photoexcitation. These constants describe the optical response of a
material and are the frequency-dependent real and imaginary parts of the complex

dielectric response of the material, n and k.

Nonlinear optics (NLO) is primarily concerned with response of a dielectric material to a

strong electromagnetic field. The electric field associated with incident light induces a

3

polarization P in the material of interest which is, in the simplest case, linearly related to
the amplitude of the electric field by the proportionality constant x. For sufficiently large
electric fields, however, the general observation has been that the microscopic charges
comprising matter fail to respond to the applied field in a strictly linear way. The

polarization can more generally be expressed in expanded powers of E,“’21
at:
P = P0 ”(1)531“ ”(2)5315; +X(3)EjEkEI [1]

where P is the induced polarization, P0 is the permanent polarization, x“) is the linear

(3’ are the second and third order nonlinear susceptibilities,

optical susceptibility, x0) and x
respectively. All-optical nonlinear optical phenomena are described by the fourth-rank

tensor 1"”.

The macroscopic NLO behavior of organic conjugated polymers originates from the
polarization response of the n-electrons. The molecular polarization behavior can be

expressed “11.21

g, = 11,.” + al-J-EJ- + flijkEJ-Ek + yWEJ-EkE, [2]

where u; is the dipole moment of the molecule induced by an electric field, 111° is the
permanent dipole moment, on,- is the polarizability, Bijk and mu are the first and second

hyperpolarizabilities, respectively. When molecules are immobile in the applied electrical

4

or optical fields (which is the case with most solid samples) )6” results directly fiom
molecular y. Thus optimizing the molecular structure for large 7 results in optimization of

x‘” of solid NLO materials.

It is important to make the distinction between resonant and nonresonant nonlinear optical
effects. Resonant effects are large and are the direct result of elementary excitations
created in a material by the absorption of incident light. Excitations so created are
considered “real” in the sense that they have a discrete lifetime and result in the permanent
exchange of energy out of the light field and into the material. Nonresonant nonlinear
optical effects are smaller in magnitude and differ in that elementary excitations are
created only “virtually” and hence no light is absorbed. These excitations result from the
spontaneous distortion of the electronic wave fimctions in the presence of the laser field.
Once the field is turned off, the electrons revert to their unperturbed configuration.
Nonresonant response times are necessarily limited by the duration of the incident electric

field.

All-optical switching devices are required to operate at very high speeds and need to have
switching speeds (both on and off) of less than ~10ps if they are to compete with existing
electronic devices. Resonant optical nonlinearities have, so far, failed to provide a
satisfactory mechanism, despite extensive investigation. There are several reasons for this,
which include the saturation of the resonant nonlinearity, the relatively slow recovery of

the nonlinearity due to the population of stationary states and heating associated with

5
absorption, which introduces undesirable thermal effects and eventually can lead to

material degradation.

However, nonresonant optical nonlinearities have been employed for all-optical switching.
Nonresonant nonlinearities are, in general, several orders of magnitude smaller than
resonant nonlinearities. If they are to be employed in all-optical switching, then they
require an efficient switching mechanism such as inverse Raman scattering. All materials
exhibit a third order nonlinear optical response.“’21 The power of the optical field required
to observe these effects depends on the nature of the electronic structure of the medium,
its dynamical behavior, symmetry and geometrical arrangements in the medium. From the
device point of view, important nonlinear optical materials are in solid form and must meet
a variety of requirements for practical use. In general, they have to have stability with
respect to ambient conditions and high intensity light sources. These materials will have to

meet many processing requirements as well.

Recently the scope of synthesis and characterization of third order nonlinear materials has
expanded tremendously. The incentive for this increased interest has been a search for a
basic understanding of the “structure-property” relationships for third order optical
nonlinearities and the technological interest in all-optical signal processing provided by
third order processes. Although studies of third order optical nonlinearities go back as far
as the 19605, this field has shown only limited progress. Hermann et. all” conducted the
first systematic investigation of third order optical nonlinearity in a conjugated system on
trans B-carotene. This work inspired considerable interest in conjugated systems and

resulted in a theoretical analysis by Rustagi and Ducuing,” who predicted the importance

6

of 1c conjugation in determining the third order nonlinear optical properties. Sauteret et.
elm reported the first investigation of the polydiacetylene PTS where they studied third
harmonic generation. This work clearly showed that the value of x“) depends strongly on
the extent of it conjugation in the nonlinear chromophore. As a result, conjugated

(3’ organic materials.

polymers have emerged as the most widely studied class of x
No symmetry requirement exists for a material to exhibit a third order nonlinear response.
All kinds of materials are potential candidates: liquids, thin films, glasses, and crystals. In
the solid state one can use various forms such as crystalline materials, amorphous
polymers, glasses and Langmuir-Blodgett films, for example amorphous polymers or
glasses are particularly usefirl media for third order nonlinear processes because they can

be readily processed into device structures.

Polydiacetylenes are one of the most widely investigated class of conjugated polymers for

their third order nonlinear optical effects. The general structure of this polymer is,

 

7
Polydiacetylenes (PDA) are prepared by the solid state polymerization of the

corresponding monomersm A large variety of PDAs with different properties can be
prepared depending on the nature of the side groups R1 and R2. PDAs offer significant
opportunities to study conformational effects, effects of conjugation length and the role of
the side groups on third order nonlinearities. Polydiacetylenes have been extensively

[7'1” with an ultrafast response

studied for their large nonlinear optical susceptibilities
time‘s] The quasi-one dimensional conjugated backbone structures coupled with chemical
identity of the side groups (R1 and R2) are responsible for these unique characteristics. The

“0’1" solution

x“) values for a large number of PDAs have been investigated in crystals,
cast film,[m Langmuir-Blodgett filmsfm solutionsm] and spin cast filmsm] by using

different measurement techniques.

8
Literature Cited

1. P. N. Prasad, D. J. Williams, Introduction to Nonlinear Optical effects in Molecules and
Polymers, Wiley Interscience, 1991.

2. Y. R. Shen Principles of Nonlinear Optics, Mley Interscience, 1984.
3. J. D. Hermann, D. Richard, Ducuing J ., Appl. Phys. Lett, 23, 178( 1973).
4. K. C. Rustagi, Ducuing J ., Opt. Commun, 10, 258(1974).

5. C. Saureret, J. P. Hermann, R. Frey, F. Pradere, J. Ducuing, R. H. Baughman, R. R.
Chance, Phys. Rev. Lett., 36, 956(1976).

6. M. Schott, G. Wegner in D. S. Chemla, J. Zyss (Eds), Nonlinear Optical Properties of
Organic Molecules and Crystals, Vol. 2 Academic, New York, 1987, p.3.

7. Polydiacetylenes, NATO ASI series B, Vol. 102, Edited by D. Bloor, R. Chance
(Nijhoff, Dordrecht, 1985).

8. G. M. Carter, J. Opt. Soc. Am. B, 4, 1018(1987).

9. T. Kanetake, T. Hasegawa, K. Ishikawa, T. Koda, T. Takeda, M. Hasegawa, K.
Kunodera, H. Kobayashi, Appl. Phys. Lett, 54, 2287(1989).

10. G. M. Carter, Y. G. Chen, M. F. Rubner, D. J. Sandman, M. K. Thakur, S. D.
Tripathy in D. S Chemla, J. Zyss (Eds), Nonlinear Optical Properties of Organic
Molecules and crystals, Vol. 2, Academic, New York, 1987, p. 85.

11. D. N. Rao, P. Chopra, S. K. Ghodhal, J. S. Swiatkievvicz, P. N. Prasad, J. Chem.
Phys., 84, 7049(1986).

12. G. Y. Berkvic, Y. R. Shen, P. N. Prasad, J. Chem. Phys., 87, 1897(1987).

13. F. Kajzar, J. Messier in D. S. Chemla, J. Zyss (Eds) , Nonlinear Optical Properties of
Organic Molecules and Crystals, Vol. 2 Academic, New York, 1987, p.51.

14. J. Lemoigne, A. Thierry, P. A. Chollet, F. Kajzar, J. Messier, J. Chem. Phys., 88,
6647(1988).

15. a) G. L. Baker, J. A. Shelburne HI, P. D. Townsend, Materials for Linear and
nonlinear Optics, Institute of Physics Conference Series, 103, 227(1990).
b) J. S. Patel, D. S. Lee, G. L. Baker, J. S. Shelburne, Appl. Phys. Lett., 56, 131(1990).

CHAPTER 1

Synchronous Pumping of Two Dye Lasers Using
a Single UV Excitation Source

Abstract
The synchronous pumping of two cavity dumped CW dye lasers operating in the
wavelength range of 405 nm to 555 nm with a single mode-locked UV pump source has
been studied. The pulses obtained from each dye laser are described quantitatively by
using the noise burst model. The cross correlation between the two laser pulse trains

indicates that the relative temporal stability is approximately one pulsewidth.

l 0
1.1. Introduction

The synchronously pumped dye laser” has proven to be an extremely usefirl light source
for a wide range of time and frequency domain spectrosc0pies.m The primary advantage
of synchronous pumping compared to passive mode locking techniques is the inherent
timing stability attainable with gain modulation. Synchronously pumped dye lasers can be
tuned over a much wider frequency range than passively mode locked dye lasers, and in
addition, it is possible to synchronize two or more dye lasers using a single pump source“)
The output pulse characteristics of synchronously pumped dye lasers have been examined
extensively, and it is generally recognized that, without the addition of a bandwidth
restricting element to the dye laser cavity, it is not possible to achieve transform limited
behaviorwl The pulses produced by the synchronously pumped dye laser have been
described quantitatively using a "noise burst" model,‘61 and, operating under optimum
conditions, it is possible to achieve pulses that are well within a factor of two of the
transform limit. Pulses that are not transform limited pose a serious problem only for

experiments where shot noise limited behavior is required or where a coherent process,

such as vibrational dephasing, is under investigation.

The excitation sources used most widely for the synchronously pumped CW dye laser are
mode locked ion lasers or frequency doubled CW mode locked Nd3+2YAG lasers. These
pump lasers operate either in the green or the red and, as a consequence, the practical blue
limit for the simultaneous operation of more than one synchronously pumped dye laser is

~550 nm. While the operation of two dye lasers below 550 nm is possible using the mode-

11
locked blue or violet lines of ion lasers as a pump source,"’ the wavelengths and low

efiiciency of these lines limit the tuning range available. Recent advances in Nd3+zYAG
laser technology have, however, allowed these limitations to be lifted and the tuning range
for simultaneous operation of two dye lasers can be extended to wavelengths as low as
405 nm. The third harmonic of a CW mode locked Nd3+zYAG laser was used to pump
two cavity dumped dye lasers synchronously. These dye lasers are independently tunable
between 405 nm and 555 nm using five different laser dyes. Conversion efficiency for
these lasers lies in the range of 1% to 25%, depending on the dye and wavelength of

operation.

1.2. Experimental

1.2.1. Source laser. The pump laser for these experiments is a CW mode locked
Nd3+zYAG laser (Coherent Antares 76-S). The output of this laser is ~30 W average
power at 1.064 pm, with ~100 ps pulses at 76 MHz repetition rate. The 1.064 um output
is frequency doubled using a type I temperature tuned LBO crystal (7 mm) to produce ~3
W average power at 532 nm, with the same pulse characteristics as for the fundamental.
The residual fundamental and collinear second harmonic light are mixed in an angle tuned
Type I BBO crystal (7 mm) to produce 2 1 W average power at 355 nm, again with the
same pulse width and repetition rate as the fundamental. Stability of this source is > 6

hours once thermal equilibrium is established within the laser head.

12
1.2.2. Dye lasers. The third harmonic output of the source laser was divided using a

~50% beam splitter and routed to two cavity dumped dye lasers (Coherent 702) operating
with three plate birefringent filters and no saturable loss. The dye circulating in each laser
was cooled to ~2°C to increase the viscosity of the dye solutions and to reduce the rate of
thermal degradation of the dyes. Low frequency noise present on the output of the dye
lasers was reduced noticeably as a result of cooling the dye solutions. Five dyes were used
to achieve the tuning range reported in here. Stilbene 1 (405 nm - 450 nm), stilbene 420
(425 nm - 470 nm), coumarin 490 (480 nm - 525 nm) and coumarin 500 (505 nm - 555
nm) were purchased from Exciton Chemical Co. Coumarin 460 (460 - 490 nm) was
purchased from Aldrich Chemical Co. For all dyes except stilbene 1 either benzyl alcohol
or methanol was used as the premix and ethylene glycol was used as the carrier solvent.
Stilbene l was dissolved directly in warm ethylene glycol. It was necessary to use three
difi‘erent mirror sets for the dye lasers to cover the wavelength range between 405 nm and
555 nm. The dye lasers were cavity dumped at ~8 MHz instead of the more commonly
used 4 MHz. Cavity dumping at 4 MHz yielded autocorrelation traces with obvious
substructure even at the synchronous cavity length, while a cavity dumping rate of 8 MHz
produced smooth autocorrelation traces without excess depletion of cavity gain. The

single pass gain for the coumarin dyes was insufficient to allow CW (76 MHz) operation.

1.2.3. Time domain measurements. Background-free, noncollinear autocorrelation
traces were recorded on a storage oscilloscope fiom the output of a scanning
autocorrelator (Spectra-Physics model 409) modified for operation between 410 and 555

nm. The modifications consisted of using two different BBO SHG crystals cut for type I

13
second harmonic generation of 450 nm and 532 nm fundamental light, replacement of the

photomultiplier with a solar blind detector (Hamarnatsu R166) and use of interference
filters (Inrad) to block the laser fundamental and pass the second harmonic to the detector.
Calibration of the time scale was accomplished by using a calibrating etalon mounted in
the autocorrelator. The hard copy output fiom the storage oscilloscope was subsequently

digitized with a commercial hardware and software package (Jandel Scientific).

Noncollinear cross correlation measurements were performed in a different manner. The
cross correlation was measured as a pump-probe signal by varying unidirectionally the
differential arrival time of the two dye laser pulses at a (type 1) BBC sum frequency
generation crystal. Sum frequency light was detected as a firnction of time delay between
the two pulses using radio frequency modulation/synchronous demodulation technology”)
The output signal from the experiment was sent directly to a computer and no
intermediate digitization steps were required. Because of the unidirectional nature of the
measurement scheme the width of the cross correlation trace is a direct measure of the

convolution of the two laser pulse envelopes.

1.2.4. Frequency domain measurements. The frequency spectra of the dye lasers were
measured by using a Heath EU-700 0.3 m scanning monochromator with 2um slits. The
band pass of this monochromator is s 0.03 nm. The output of the PMT detector was

converted to a voltage and output to an X-Y recorder, and this trace was digitized.

14
1.4. Results and Discussion

The fitndamental wavelength range accessible to synchronously pumped dye lasers is
limited by the wavelength of the source laser. Extending the tuning range of this class of
laser further to the blue would allow access to a wide range of interesting chemistry and
physics. Earlier reports have shown that either the mode locked UV lines of an Ar+ or
Kr+ laser,[9’1°] or the third harmonic of a mode locked Nd3+:YAG laserm] can be used to
excite a single dye laser. There is no report on the literature where a single CW mode
locked UV source has been used to pump two dye lasers simultaneously. It was found that
400 to 500 mW of average UV power is sufficient to pump each dye laser, and the tuning
curves for five dyes spanning the wavelength range between 405 nm and 555 nm are
shown in Figure 1. We note that the efficiencies for the stilbene dyes are substantially
higher than those for the coumarins. In constructing these gain curves we attempted
operation with several other coumarin dyes, but their thresholds for laser operation were
higher than the ~400-500 mW average power available to each dye laser. We also
demonstrated operation with coumarin 450 but have not included its gain curve here
because stilbene 420 offers coverage of the same wavelength range with higher conversion

efficiency.

The temporal and frequency profiles of pulses produced from the coumarin 460 and
stilbene 420 lasers determine collectively how close their performance is to the transform

limit. These data are summarized in Table 1.1. Figure 1.2a shows a representative

15

stilbene 1

22; /

\3\D stilbene 420

1:51 I” .5

 

 

8‘
if»
air?
0 12 — \n
g . A
‘2‘ 10 — \D
E .
8 8 ' coumarin 460
X 6 _ / coumarin 490
v
’ V vhv.e°'. coumarin 500
4 - A \.
.. . ../++\
2 "' ++ \ +
/ . \+
' + \
0.141.11111.1.1.+1
400 420 440 460 480 500 520 540 560

wavelength (nm)

Figure 1.1. Gain curves for five laser dyes spanning the wavelength range between 405 nm
and 555 nm. Conversion efficiency is defined here as the ratio of the input
average power to the output average power.

1.0 - a
0.8 -
0.6 -

1' V

0.4

intensity (a. u.)

0.2

I

 

0 O 1 4L - l 4 J l A I

22700 22710 22720 22730 22740 22750
frequency (cm'l)

1.0 -

0.8 -

0.6 r

intensity (3. u.)
C
A
l

0.2 -

 

4‘—

J

 

I n I l J
20600 20610 20620 20630 20640

frequency (cm'l)

Figure 1.2. (a) Frequency spectrum for a stilbene 420 dye laser operating at 440 nm
presented with a best fit gaussian fiinction for the spectrum. (b) Frequency
spectrum for a coumarine 460 dye laser operating at 485 nm, presented with a
best fit gaussian firnction.

Table 1.1. Output parameters of stilbene and coumarin dye lasers modeled by the noise

burst model.

17

 

laser dye 7. (nm) Av (cm'l) AvAtn AvAte

stilbene 420 430 4.43 0.408 0.801
" 440 4.29 0.400 0.748
" 465 4.07 0.305 0.629

coumarin 460 460 3 .02 0.340 0.466
" 473 3.11 0.321 0.489
" 485 3.10 0.302 0.490

 

frequency spectrum for operation with stilbene 420 at 440 nm and Figure 1.2b shows a
frequency spectrum for operation with coumarin 460 at 485 nm. While there are slight
deviations, these spectra are represented well by a Gaussian profile, and this fit is shown in
Figures 1.2. The corresponding autocorrelation traces for these fiequency spectra are
shown in Figures 1.3a and 1.3b. If the output of these lasers is transform limited it is
expected that the autocorrelation functions would be fit well by a Gaussian profileml
Experimentally this is not observed. However, it was found that these pulses can be
modeled quantitatively using the noise burst modelm In this model, the laser pulse is

separable into two parts; a bandwidth limited pulse of thermal radiation (noise) and a

slower temporal envelope.

E(t) = T(t)/1(1) [1]

where A(t) is the stochastic profile of the thermal radiation and T(t) is the temporal

envelope. For a pulse described by this model, an autocorrelation of the form

18

1.0 _

 

 

 

 

 

a

3 0.8:
A? 0.6 -
a .
g 0.4 4
a 0.2.
m .

0.0 ‘ ' ‘ J

—40 -20 0 20 40
delay time (ps)

1.0 -
A . b
r-i 0.8 e
3 .
g» 0.6 -
i 04'
.3: - _
E 0.2 -
m i

00 A A l 1 I 4 I AIL I I

-40 -20 0 20 40
delay time (ps)

Figure 1.3. (a) Autocorrelation trace corresponding to the frequency profile shown in
Figure 2a, presented with a fit to Equation 2. Parameters of the fits are presented
in Table I. (b) Autocorrelation trace corresponding to the frequency profile
shown in Figure 2b, presented with a fit to Equation 2.

19
G<t)=G.(t)[1+G.(t>] [21

is expected, where Ge(t) is the component of the autocorrelation arising from the function
T(t) and Gn(t) is the component of the autocorrelation determined by A(t).“3’l‘” If this
model describes the data adequately then the time bandwidth product AVA‘tn ~ 0.44
should be observed. The quantity Atn is the actual pulse width and is inferred from the
autocorrelation trace (‘/2Atn for a Gaussian). The autocorrelation data was fit to Equation
1.2 assuming a Gaussian functionality for both Ge(t) and Gn(t), and the results of these
fits are presented in Table 1.1 and Figures 1.3. For all of the wavelengths studied it was
found that the quantity AvAtn ~ 0.4 and AVA‘te ~ 0.75. The actual value of AVA‘tn varies
with wavelength and this might be due to slight deviations fi'om a Gaussian frequency
profile. The output of these lasers is clearly not transform limited, AvA‘ce > 0.4, but is
well within a factor of two of the transform limit (see Table 1.1). This deviation from
transform limited behavior is typical of the output of a synchronously pumped dye laser
where no additional bandwidth restricting elements have been introduced into the laser
cavity.[”] We expect that, if an etalon or other bandwidth restricting element were
introduced into the dye laser cavities, it would be possible to achieve transform limited

performance.”

One of the most important properties of a laser system like the one reported here is the
relative temporal stability of the pulse trains of the two dye lasers. Experimental
measurement of the cross correlation of the two laser pulse trains provides a direct

measurement of this jitter if the pulse envelope is known. The time width of the cross

20
correlation measurements for these two laser systems at a series of wavelength pairs are

shown in Table 1.2, and the cross correlation for the pulses shown in Figures 1.2 and 1.3
Table 1.2. Cross correlation widths for different dye laser wavelength combinations.

Wavelengths for the two lasers are indicated on top and side. Cross
correlation time widths are from fits of the data to a Gaussian function.

 

laser wavelength 460 nm 473 nm 485 nm
430 nm 12.6 ps 13.1 ps 14.2 ps
440 nm 12.3 ps 14.6 ps 14.7 ps
465 nm 11.6 ps 12.4 ps 11.6 ps

 

is shown in Figure 1.4. The results of these measurements demonstrate that, in general, a
cross correlation width that is on the order of twice that of the individual pulses, implying
approximately one pulsewidth of instability between the two pulse trains can be expected.
The measured jitter is consistent with results for this same system when it is pumped by
the second harmonic of the Nd3+zYAG laser and operated with red dyes, suggesting that
the relative temporal stability of the laser pulse trains is determined by the noise
characteristics of the source laser instead of the gain characteristics of the dye lasers. In
Figure 1.4, a fit of the cross correlation data to a Gaussian fiinction is shown. Because of
the unidirectional nature of the cross correlation measurement, asymmetry in the individual
laser pulses can produce asymmetry in the cross correlation. The data shown in Figure 1.4
is typical of this system; all of the cross correlation data are fit best by a symmetric

Gaussian function, suggesting the individual laser pulses are symmetric.

21

 

 

 

1.0 -

0.8 -

’5 0.6 ~
8

4?; _
2
8

a 0.4 -
CD
I

(I) ..

0.2 -

y.

-40 -20 O 20 40

delay time (ps)

Figure 1.4. Unidirectional cross correlation for the two pulses detailed in Figures 2 and 3
together with a fit of the data to symmetric Gaussian firnction.

22
1.5. Conclusions

The operation of two CW synchronously pumped cavity dumped dye lasers with a single
UV excitation source has been described. This advance extends the fundamental tuning
range of this class of lasers to cover the spectrum between 405 nm and ~ lum with no
gaps. The pulse characteristics of this system are described quantitatively by the noise
burst model and are consistent with other synchronously pumped dye lasers operating at
longer wavelengths. Instability between the dye laser pulse trains is on the order of one
pulsewidth, demonstrating that pump-probe measurements previously restricted to
wavelengths longer than ~550 nm can be performed in the blue. It is expected that, as with
other synchronously pumped systems, the addition of saturable loss to the dye lasers will
result in better absolute time resolution. This extended spectral range has been used to
study the ultrafast stimulated emission response of perylene in dilute solutionm] and for
the study of excitation migration in conjugated polymers (see chapter 2 and 3)“‘” and
stimulated inverse Raman scattering studies of the 1‘” response of poly(4BCMU) (see

chapters 4 and 5).“7’18]

23
Literature Cited

1. J. M. Harris, R. W. Chrisman and F. E. Lytle, Appl. Phys. Lett., 26, 16 (1975).

2. see "Chemical Applications of Ultrafast Spectroscopy", by G. R. Fleming, Oxford,
1986, and references therein.

U)

. (a) R. K. Jain and J. P. Heritage, Appl. Phys. Lett., 32, 41 (1978). (b) J. Kuhl and D.
von der Linde in "Picosecond Phenomena III", ed. by K. B. Eisenthal, R. M.
Hochstrasser, W. Kaiser and A. Laubereau, Springer, Berlin, 1982, p.201.

4. G. J. Blanchard and M. J. Wirth, Opt. Commun, 53, 394 (1985).

LII

. B. F. Levine and C. G. Bethea, IEEE J. Quant. Electron, QE-l6, 85 (1980).

O\

. H. A. Pike and M. Hercher, J. Appl. Phys., 41, 4562 (1970).

7. E. P. Economou, R. R. Freeman, J. P. Heritage and P. F. Liao, Appl. Phys. Lett., 36,
21 (1980).

8. P. Bado, S. B. Wilson and K. R. Wilson, Rev. Sci. Instrum, 53, 706 (1982); L. Andor,
A. Lorincz, J. Siemion, D. D. Smith and S. A. Rice, Rev. Sci. Instrum, 55, 64 (1984);
G. J. Blanchard and M. J. Wirth, Anal. Chem, 58, 532 (1986).

9. J. N. Eckstein, A. I. Ferguson, T. W. Hansch, C. A. Minard and C. K. Chen, Opt.
Commun, 27, 466 (1978).

10. A. I. Ferguson, Opt. Commun, 38, 387 (1981).

11. T. P. Carter, private communication.

12. K. L. Sala, G. A. Kenney-Wallace and G. E. Hall, IEEE J. Quant. Electron, QE-16,
990 (1980).

13. D. B. McDonald, D. Waldeck and G. R. Fleming, Opt. Commun, 34, 127 (1980).

14. D. B. McDonald, J. L. Rossel and G. R. Fleming, IEEE J. Quant. Electron, QE-17,
1134 (1981).

15. S. A. Hambir, Y. Jiang and G. J. Blanchard, J. Chem. Phys., 98, 6075(1993).

16. S. A. Hambir, T. Yang, G. J. Blanchard and G. L. Baker, Chem. Phys. Lett., 201,
521(1993).

17. S. A. Hambir, G. J. Blanchard and G. L. Baker, J. Chem. Phys., 102, 2295(1995).

18. S. A. Hambir, D. Wolfe, G. J. Blanchard and G. L. Baker, in preparation.

CHAPTER 2

Excitation Migration in the Polydiacetylene DCHD

Abstract

Ultrafast spectroscopic results on excitation migration in the polydiacetylene DCHD, a
crystalline conjugated polymer with carbazole side groups has been investigated. It was
found that excitations created initially on the carbazole side group migrate rapidly (310 ps)
to the polydiacetylene backbone. Relaxation of this excitation has the same spectral and
temporal signature as relaxation of the backbone excitonic resonance. Comparison of
these data to the electroabsorption response for DCHD confirms excitation transfer and
not carrier transfer. The rapid excitation transfer measured is consistent with a Forster

through-space mechanism. The excitonic transition localized to the polymer backbone

decays nonradiatively into side group vibrational modes that have a characteristic (e‘l)

lifetime of ~3.3 ns.

24

25

2.1. Introduction

Conjugated polymers have been studied extensively over the past decade because of their
characteristically large third order nonlinear optical response, x(3).m This response is
thought to originate from the extensively delocalized 7t electron system found in
conjugated polymers, but the molecular level understanding of the hyperpolarizability
responsible for the observed optical nonlinearity is incomplete. A significant experimental
limitation to gaining this understanding is the disorder present in many conjugated
polymers. Most conjugated polymers are disordered to the extent that it is difficult to
distinguish between inter-chain effects and intra-chain effects. One class of conjugated
polymers, the polydiacetylenes, can be synthesized as large single crystals, where inter-
chain and intra-chain effects can be effectively distinguished. The side groups of
polydiacetylenes determine their morphological properties and, for crystalline forms,
separate the individual polymer backbones from one another so that these materials behave
as model l-dimensional systemsm Despite these advantageous materials properties, our
understanding of the dynamics of optical excitation and subsequent relaxation in these

systems remains limited.

Poly[1,6-di(N-carbazolyl)-2,4-hexadiyne],[36] (DCHD, Figure 2.1) a polydiacetylene with
N-methylcarbazole side groups was studied to understand the role that these side groups

play in optical energy storage, excitation migration and relaxation in polydiacetylenes.

26

4.13 eV

\ (300 nm)
/

Q
0
U @ €32; 2X.)
(1022 3):.) O
H Q
Q

Figure 2.1. Structure of poly[1,6-di (N-carbazolyl)-2,4-hexadiyne] (DCHD). The long axis
of the carbazole side group is tilted 81° and the short axis 43.3 ° with respect to
polymer backbone in the crystal. The arrows indicate the transition dipole
moment directions.

27
DCHD was chosen for several reasons. First, it possesses the lowest energy excitonic

transition of any crystalline polydiacetylene reported to date,[31 implying a stable and highly
ordered polydiacetylene backbone, and second, its side groups can be excited optically,”
allowing the direct study of excitation transfer between the polymer side groups and
backbone. In addition, the photoconductivity of DCHD has been reported to be
substantially greater than that seen in other polydiacetylenes, suggesting the possible role
of the carbazole side groups in carrier injection onto the polymer backbonem The
carbazole side group possesses two allowed transitions at energies in the near UV, one
polarized along the chromophore short axis (3.57 eV) and the other polarized along the
chromophore long axis (4.13 eV).”1 It was found that, for excitation of either side group
transition, the excitation migrates to the polymer backbone in less than the 10 ps,
experimental time resolution of the spectrometer. Further, the excitonic transition
localized on the polymer backbone relaxes rapidly (310 ps) and a slow, energy-dependent
response is observed. The energy dependence of this slow response is the same, within
experimental uncertainty, regardless of how the excitation is produced initially. Because
this slow response differs from the electroabsorption response for DCHD,m the data
presented here are consistent with excitation transfer rather than carrier injection from the
polymer side group to the backbone. The slow response arises from dipolar coupling
between side groups and the conjugated diacetylene backbone, analogous to that seen

previously for PTS, a polydiacetylene with p-toluene sulfonate side groups.[8’9]

28
2.2. Experimental

Polydiacetylene DCHD was prepared by exposing crystalline 1,6-di(N-carbazolyl)-2,4
hexadiyne to 50 Mrad of 60Co y-radiationm The resulting polymer crystals were typically
several microns thick. Thin crystalline sheets of DCHD were separated from the crystals
using masking tape, deposited on Pyrex substrates and washed with tetrahydrofirran to
remove residual adhesive from the delamination operation. The resulting crystals were
estimated to be several hundred A thick. The linear absorption spectrum of a somewhat
thicker crystal of DCHD is presented in Figure 2.2. The thickness of this crystal was

estimated to be 1600A, based on the value of “max =5x105 cm'1 .[41

Ultrafast pump-probe spectroscopic measurements were made on DCHD by using a
spectrometer that was described in detail in chapter 1.“°] Briefly, a mode-locked Nd:YAG
laser (Coherent Antares 76-S) was used to produce 100 ps pulses at 76 MHz repetition
rate. The second harmonic of output from this laser (2W average, 532 nm) was used to
pump synchronously two cavity dumped dye lasers (Coherent 701-3) operating with either
LDS698 or DCM laser dyes (Exciton). The output of the pump laser was ~150 mW
average power at 8 MHz repetition rate, 7 ps pulse width. The probe laser output was ~50
mW average power at 8 MHz repetition rate, 5 ps pulse width. A typical cross correlation
measurement yielded a 10 ps FWHM response. For experiments pumping the carbazole
side groups, the pump laser pulse train was frequency doubled using a 2 mm angle tuned
LiIO3 SHG crystal. Average pump power at the sample was _<. 1 mW for all experiments,

and the average probe power was ~150 uW. Detection of all pump-probe signals was

29

3.5 -
3.0 3 /' "\
.. / .\.

2.5 - \
2.0 - / x'\

1.5- / °

1.0- /

Absorbance

 

0'0 T I ' I ' I I I ' fl
1.75 1.80 1.85 1.90 1.95 2.00

Energy, eV

 

Figure 2.2. Linear absorption spectrum of a ~1600A thick crystal of DCHD in the region
of the polymer backbone excitonic transition.

30
accomplished by using a radio and audio frequency triple modulation, shot noise limited

detection system.“”

2.3. Results and Discussion

Much of the work on polydiacetylenes has focused on the optical properties of the
polymer backbone because it is thought to be the dominant linear and nonlinear
chromophore. The role that the polymer side group plays in the material optical response
can be profound, however. For example, crystalline polydiacetylene poly(4BCMU) has an
excitonic resonance at 1.95 eV. Unlike most other polydiacetylenes, this crystalline
polymer can be disordered to a limited extent or dissolved because of the labile nature of
its side groups. Exposing poly(4BCMU) crystals to solvent vapors or heat causes a 0.3 eV
blue shift and broadening of the excitonic resonance due to increased disorder on the
length scale of the excitonml Even for rigid crystalline systems, the side groups play an
important role in energy relaxation subsequent to optical excitation.[8’9] For PTS, a

"3] and there is a slow energy-

crystalline polydiacetylene, the exciton lifetime is ~2 ps
dependent optical response subsequent to excitonic decay.[8’9] The optical energy
introduced in the form of an exciton is dissipated vibrationally, with the dominant
relaxation pathway being through the side groups. Because the side groups possess a
significant permanent dipole moment they are coupled strongly to the side groups of

neighboring polydiacetylene chains. Vibrational motions on side groups from one chain

couple to side groups on adjacent chains, which in turn modulate the energy of the

3 1
backbone 1t system to which they are attached. Thus the identity of the polymer side group

can play a deterministic role in the observed optical response of the polymer backbone.

The optical response of the polymer backbone of the polydiacetylene DCHD following
excitation of the carbazole side groups and direct excitation of the excitonic transition has
been studied. It was found that the excitonic transition responds identically to each of
these different modes of excitation. Excitation transport fi'om the side group to the
backbone occurs in less than 10 ps. These data are consistent with the excitation transfer
proceeding by a Ferster through-space mechanism. The reasons for this assertion are

detailed below.

Relaxation of the excitation on the polymer backbone involves several steps, with
markedly different time constants. Figures 2.3-2.5 show the energy dependence of the fast
and slow responses of the excitonic absorption of DCHD subsequent to direct backbone
excitation and side group excitation. The limited spectral ranges reported for excitation of
the side groups, presented in Figures 2.4 and 2.5, reflect the small signal size encountered
in these experiments. Over the spectral ranges examined, however, the slow response is
the same for all modes of optical excitation that were used. In all experiments the fast
response arises from a uniform bleaching of the excitonic transition due to a phase-space
filling mechanism?” The raw data were recorded as time scans at a series of fixed pump
and probe laser energies. The spectral responses presented in Figures 23-25 were
reconstructed fi'om individual time scans with the phase space filling response for

calibration. The zero-time responses were scaled to correspond in intensity to the linear

32

1.00 .4 fast response

 

 

 

 

I
0.75 - "\l
d / \I .
\I

#5 0.50 .. I 0 30 ps
2 . / \-
V I
a» 0.25 - ' / e ‘
<1) ‘ A " ‘\
4.;
E 0.00 1\
3 . ‘
._. ‘ e
(I)

-0.25 - \

A
-050 4 A
i J .
-0.75 n
125 ps slow response
A
-1.00 I I U _ I T f I I
1.80 1.85 1.90 1.95 2.00

Energy, eV

Figure 2.3. Transient response of DCHD for excitation at 1.908 eV, direct excitation of
the exciton. The fast response (0 ps, I) is scaled to the linear absorption
spectrum. Slow reponses are shown for 30 ps (0) and 125 ps (A) delay times.

33

1.00- fast response
" .- /.-—I \
0.754 ( 0 PS) \

.9
Ur
O
1
I

 

E e 30 ps

£9 025- /

i . 3 . ,
.3 000 “(J /‘/\.\1 h
is” \/ ‘ I l

 

 

—O,25-4
slow response \\/0"\
-0.50‘
‘ \f 125
ps
\‘
-0.75 .,.,.,,T,,
1.850 1.875 1.900 1.925 1.950 1.975

Energy, eV

Figure 2.4. Transient fast (0 ps, I) and slow (30 ps, 0; 125 ps, A) responses of the
DCHD excitonic transitions for excitation at 4.13 eV, the long axis polarized
transition localized on the carbazole side group.

34

1.00 F fast response

(0 p8) I/II‘. \..

 

0 75" \-
. \.\..
0.50 -
g 0 25 _ 30 ps
Cd /.\
a? 000
J 471/
'3 -025 - \
.3

-050 - \°/ ,/ .25..

slow response

.. J\ .

A

-0.75 -

 

-l.00 '
1.850 1.875 1.900 1.925 1.950

Energy, eV

Figure 2.5. Transient fast (0 ps, I) and slow (30 ps, 0; 125 ps, A) responses of the
DCHD excitonic transition for excitation at 3.57 eV, the short axis polarized
transition localized on the carbazole side group.

35
absorption spectrum because the samples used were not sufficiently uniform to detemrine

absolute signal intensities. The slow response is attributed to modulation of the backbone
excitonic transition by side group vibrational motion. This response has the same physical

origin as the slow relaxation seen in the polydiacetylene PTSM but for DCHD the time
constant of the exponential population decay was measured to be 3.3 :t 0.3 us at 300 K
(Figure 2.6). The corresponding relaxation time for PTS is 130 ps.” This difference is
attributed to the density of bath mode states available to the different side groups in these

two crystalline polydiacetylenes.

The ultrafast energy transfer from the carbazole side group to the polydiacetylene
backbone is the same for transfer from either of the two accessible transitions localized on
the side groups. The two side group transitions occur at 3.57 eV and 4.13 eV, and are
polarized along the carbazole short axis and long axis, respectively. The carbazole side
group is planar and its long and short axes make angles of 81° and 433°, respectively,
with the polydiacetylene backbone“) This geometric arrangement permits, energy transfer
to occur via a Forster mechanism. The rate constant for Forster energy transfer is given

by,[15]

3x2R8
27DR6

 

kDA = [1]

where TD is the lifetime of the excited carbazole chromophore in the absence of the

polydiacetylene backbone. For N-methylcarbazole, tfl=l8.3 nsm R0 is the critical

36

1.0 -

0.5 -

 

 

.0
o

 

 

signal intensity (a. u.)
i:
LII
r

 

 

 

 

 

 

 

_1.5 . r I 1 1l 1 r . 1 . r
0 200 400 600 800 1000

delay time (ps)

Figure 2.6. Time-scan of the pump-probe signal for excitation at 1.908 eV and probing at
1.879 eV. The magnitude of the slow absorptive signal decays exponentialy with
a time constant of 3.3 :i: 0.3 ns.

37
transfer distance, a quantity related to the spectral overlap between the donor and

acceptor chromophores and the donor fluorescence quantum yield?” For the 3.57 eV
transition R0 was calculated to be 26A and for the 4.13 eV transition R0 was calculated to
be 29A, the difference arising fiom a D4 dependence in the spectral overlap term. From
X-ray crystallography data, R was estimated to be 3.26A .[61 The term x2 in Equation 1,

the geometry factor, is given by

K2 = (sin OD sin 6A cos¢ — 2cos 00 cos 6A) [2]

The terms 0 are the angles of the transition dipole moments for the donor and acceptor
with respect to the vector joining the two chromophores, and d) is the azimuthal angle
between them. 1:2 was calculated to be 0.16 for the short axis polarized transition (3.57
eV) and 0.10 for the long axis polarized transition based on the X-ray crystal structure of
DCHD“) Thus we expect kD A ~ 6.5 x 1012 s'1 for the carbazole short axis transition and
kD A ~ 2 x 1012 s‘1 for the long axis transition. The experimental observation that the
excitation transfer proceeds in :10 ps is consistent with dipolar coupling being the

dominant mechanism.

The slow response observed for DCHD exhibits both an energy dependence and a time-
dependence. These two features will be addressed separately. Before the possible origins
of the slow response are discussed, however, it is important to compare the observed

energy dependence to other data on DCHD. DCHD is known to exhibit a high

38
photoconductivity compared to other polydiacetylenes.” This high photoconductivity is

thought to be related to carrier injection onto the backbone from the side groups. The
spectral response of a polymer backbone perturbed by the presence of a local electric field
is seen in electroabsorption experiments. Kawabe et al. have reported the
electroabsorption spectrum for DCHD,m and their data has been compared to the slow
response in Figure 2.7. Clearly the slow response reported here does not correspond to an
electroabsorption signal for DCHD. Excitation of the carbazole side groups with little
excess spectroscopic energy does not produce a carrier population on the polymer

backbone. These data are dominated by excitation transfer rather than carrier injection.

The energy dependence of the slow response is strongly suggestive of substructure within
the exciton envelope. Modulation of the polydiacetylene backbone by sidegroup
vibrational motions will produce either a blue-shift or a red-shift of the excitonic
resonance, just as has been reported for PTS.[8’9] If the excitonic response in DCHD was
comprised of a single feature one would expect a single zero-crossing in the energy
dependence. For such a feature, a blue-shift would produce a transmissive response for
energies below the excitonic maximum and an absorptive response at energies higher than
the maximum. The several zero-crossings observed in the slow response indicate the
presence of more than one feature. The character of this response can be reproduced by
shifting three overlapped Lorentzian lines, although agreement with this simple model is
by no means proof of this mechanism. It would be expected to find evidence for excitonic
substructure in the linear response of DCHD (Figure 2.1). There are several factors that

obviate this possibility, however. Near the excitonic maximum, the imaginary part of the

0.75

0.50 ,-

.0
N
Ur

0.00

-0.25

signal intensity ( a. u.)

050

-0.75

-1.00

1.75

39

_ electroabsorption

slow response
e

.‘ /e’\
r?

 

 

 

11.01.11

1 I
1.80 1.85 1.90 1.95 2.00
Energy, eV

A

Figure 2.7. Comaprison of the slow response reported here (solid line with data points)
with the electroabsorption response of DCHD, taken from reference 7 (dotted
line). Signal intensity scales are arbitrary.

40
complex dielectric response of crystalline polydiacetylenes is e z 25. For systems with

such a high dielectric response the linear absorption spectrum is not a direct measure of
the intrinsic spectral linewidthlzl Attempts to infer underlying line shapes from absorption
measurements can thus be misleading. The second problem with this approach is that the
observed linear response for DCHD is essentially identical to the room temperature
absorption spectrum of PTS?“ The room temperature slow response of PTS exhibits a
markedly different spectral signature, however, indicating that there is not a simple
correlation between the excitonic absorption band shape and the slow response in these

materials.

Two explanations for the existence of more than one excitonic feature in DCHD could be
suggested. The first is the possibility of more than one allowed orientation of the carbazole
side groups within the polydiacetylene crystal. The potential energy surface for rotation of
the carbazole side groups about their tethering bond may not be smooth, and may possess
several local minima. This is not unknown in polydiacetylenes; the potential energy surface
for side group rotation in PTS exhibits minima at i8° from its room temperature
equilibrium position, and the barrier heights for these local minima are ~200 K. Similarly,
energy deposited into the side groups fi'om excitonic relaxation may be sufficient to
overcome a side group rotation barrier in DCHD. It is also possible that the second
feature corresponds to partially oxidized DCHD. Oxygen-induced spectral features in PTS

“7] and, because of the thin films studied, any oxidized material on the surface

are known
of the crystal could contribute significantly to the overall spectral response. Obtaining the

x-ray crystal structure of the crystals used for the optical experiments would allow the

41
evaluation of these possibilities. Unfortunately, the crystalline domain sizes of these thin

crystals are too small for x-ray diffraction measurements to be performed. The second
feature in the slow response is its time evolution. Although subtle, the response at 30 ps
delay time difl‘ers from the response at 125 ps delay (Figures 2.3-2.5). Most probably this
difference is related to the time required for energy to migrate to the vibrational modes of
the side groupsm Low temperature measurements to examine this relaxation process are

discussed in greater detail in the next chapter.

2.4. Conclusion

Ultrafast energy transfer between the side groups and backbone of a conjugated polymer
at room temperature were investigated. This effect is manifested by a transient bleaching
of the polydiacetylene backbone excitonic feature immediately following excitation. The
rate of excitation transfer is consistent with a Forster through-space mechanism. The slow
decay following the exciton bleaching response is strongly energy dependent, but is not
commensurate with electroabsorption datam The energy dependence of this slow
response is the same for excitation of either the backbone or the side group, and is
observed to "build up" in time, likely due to the rate at which vibrational energy
accumulates in the polymer side groups. While the complex nature of the slow signal
indicates more than one spectral feature within the excitonic envelope, it was not possible
to identify definitively the origin of these several features. Further experiments including
low temperature spectroscopic and x-ray diffraction measurements will reveal their origin.

It is clear from these data, however, that direct excitation of the DCHD carbazole side

42
groups with little excess spectroscopic energy does not result in carrier injection onto the

polydiacetylene backbone.

43
Literature Cited

1. Nonlinear Optical Properties of Organic Molecules and Crystals, Vol. 2, ed. by D. S.
Chemla and J. Zyss, Academic Press, 1987.

2. G. J. Blanchard, J. P. Heritage, A. C. Von Lehmen, M. K. Kelly, G. L. Baker and S.
Etemad, Phys. Rev. Lett., 63, 887 (1989).

DJ

. (a) M. E. Morrow and C. J. Eckhardt, Chem. Phys. Lett., 144, 65 (1988). (b) L.
Sebastian and G. Weiser, Phys. Rev. Lett., 46, 1156 (1981). (c) L. Sebastian and G.
Weiser, Chem. Phys., 62, 447 (1981).

4. R. J. Hood, H. Muller, C. J. Eckhardt, R. R. Chance and K. C. Yee, Chem. Phys. Lett.,
54, 295 (1978).

S. U. Seiferheld, B. Ries and H. Bassler, J. Phys. C., 16, 5189 (1983).
6. P. A. Apgar and K. C. Yee, Acta. Cryst, B34, 957 (1978).

7. Y. Kawabe, F. Jarka, N. Peyghambarian, Dandan Guo, S. Mazumdar, S. N. Dixit and F.
Kajzar, Synth. Metals, 49-50, 517 (1992).

8. G. J. Blanchard, J. P. Heritage, G. L. Baker and S. Etemad, Chem. Phys. Lett., 158,
329 (1989).

9. M. J. Nowak, G. J. Blanchard, G. L. Baker, S. Etemad and Z. G. Soos, Phys. Rev. B,
B41, 7933 (1990).

10. Y. Jiang, S. A. Hambir and G. J. Blanchard, Opt. Commun, 99, 216(1993).

11. (a) G. J. Blanchard and M. J. Wirth, Anal. Chem, 58, 532 (1986). (b) P. Bado, S. B.
Wilson and K. R. Wilson, Rev. Sci. Instrum, 53, 706 (1982). (c) L. Andor, A.
Lorincz, J. Siemion, D. D. Smith and S. A. Rice, Rev. Sci. Instrum., 55, 64 (1984).

12. M. J. Nowak, G. J. Blanchard, G. L. Baker, S. Etemad and Z. G. Soos, in Conjugated
Polymeric Materials; Opportunities in Electronics, Opto-Electronics and Molecular
Electronics, ed. by J. L. Brédas and R. R. Chance, NATO ASI Series E, 182, Kluwer
Academic Publishers, Dordrecht, 1990, 412.

13. G. M. Carter, J. V. Hryniewicz, M. K. Thakur, Y. J. Chen and S. E. Meyler, App].
Phys. Lett., 49, 998 (1986).

14. s. Schmitt-Rink, r). s. Chemla and 1). A. B. Miller, Adv. Phys., 33, 89 (1939).

15. G. R. Fleming, Chemical Applications of Ultrafast Spectroscopy, Oxford University
Press, 1986, 218-223.

44

16. I. B. Berlrnan, Handbook of Fluorescence Spectra of Aromatic Molecules, Academic
Press, 1971, p. 208.

17. G. J. Blanchard and J. P. Heritage, Chem. Phys.Lett., 177, 287 (1991).

CHAPTER 3

Low Temperature Transient Optical Spectroscopy of
Polydiacetylene DCHD. Evidence for a Distribution
of Side Group Orientations

Abstract
The transient optical response of the polydiacetylene DCHD at low temperature was
investigated. Ultrafast pump-probe spectroscopic measurements reveal that excitation of
the polymer N-methyl carbazolyl side group results in instantaneous (<10 ps) excitation
transfer to the polymer backbone at 10 K. The slow population recovery signal seen
subsequent to excitation transfer to the polymer backbone exhibits an excitation energy
dependence that is indicative of the existence of multiple conformers. The energy barriers

between these conformers is inferred to be < 0.5 kcal/mol.

45

46
3.1. Introduction

Research over the past decade on conjugated polymers has shown that this family of
materials is potentially usefirl for photonic signal processing applications“) In addition,
conjugated polymers have proven to be nearly ideal model systems for fundamental
investigations of the nonlinear response of strongly coupled three-level systemsml A
limiting problem, inherent to many studies on the optical response of polymers, has been
the ability to distinguish between interchain and intrachain relaxation processes. Several
polydiacetylenes have been used as prototypical conjugated polymers because of their high
degree of crystallinity and the substantial physical separation between individual polymer
backbones. These favorable material properties preclude the occurrence of a host of
intermolecular relaxation processes and allow direct examination of the linear and
nonlinear optical processes intrinsic to a single polymer backbone. An added advantage of
using polydiacetylenes is that the identity of the side groups can be controlled synthetically

while maintaining the same backbone structure, so that interactions between the polymer

backbone and its side groups can be evaluated systematically.

Understanding the interaction between the polymer backbone and side groups using the
polydiacetylene DCHD was the focus of this investigationwl This particular system was
chosen because it is highly crystalline and the N-methyl carbazole side groups of DCHD
can be excited selectively. In the previous chapter, it was demonstrated that excitation of
either of the DCHI) side group transitions resulted in a fast transfer of the excitation to the

polymer backbone, and subsequent slow relaxation of the backbone excitation into the

47
ground state vibrational modes of the side groupsm The relaxation of energy into the side

groups was manifested by a long-lived modulation of the backbone excitonic resonance
arising fi'om dipole-induced dipole coupling between the side group and the polymer
backbone. These results demonstrated that the side group excited electronic states are
coupled strongly to those of the polymer backbone, but two fundamental questions
remained to be answered. First, excitation transfer between the excited side group and the
polymer backbone was observed to occur in less than 10 ps at 300 K, but it was not
possible to resolve this rate. These data suggested, but did not prove, the dominance of a
Forster process. Second, the presence of sub-features within the slow relaxation envelope
of the side group vibrational motions could not be explained unambiguously for lack of
structural information and the amount of thermal energy present at 300 K. For these
reasons it was necessary to undertake a series of measurements on DCHD at 10K. The
results obtained do not clarify the nature of the fast excitation transfer between the excited
side group and the polymer backbone, but the data presented here provide insight into the
reasons for the spectral profile of the slow responses. These results show that several
subtly different conformers are present in crystalline DCHD, and the energetic barriers

between these conformers are small, i. e. <0.5kcal/mol.

3.2. Experimental

3.2.1. Materials and Temperature Control. Polydiacetylene DCHD was polymerized

from crystalline l,6-di(N-carbazolyl)-2,4-hexadiyne monomer with 50 Mrad of “Co 7-

48
radiationm Sheets of crystalline DCHD polymer were delaminated from thick (~100 um)

crystals using masking tape and were subsequently transferred to a pyrex substrate. The
supported thin crystals (3 1000A) were washed with tetrahydrofuran to remove any
remaining adhesive from the crystal surface. The pyrex substrate was mounted in a flowing
He(]) cryostat (Janis ST-lOO). The absorption spectra of DCHD at 300 K and 10K were
obtained from their corresponding reflectance spectra by means of Kramers-Kronig

[9-11]

transformation. For these reflection measurements, thick DCHD crystals were

mounted in the cryostat. These data are presented in Figure 3.1a and 3.1b.

3.2.2. Pump-probe spectroscopy: The pump-probe measurements presented in this
chapter can be divided into two classes; direct excitation of the polymer backbone and
interrogation of its excitonic optical response, and excitation of the polymer side group
followed by interrogation of the backbone excitonic response. Only a brief description of
the pump-probe spectrometer will be given here because it has been described in detail
chapter 1.112] A CW mode locked Nd:YAG laser (Coherent Antares 76-S) produces 30W
average power at 1064 nm with 100 ps pulses at 76 MHz repetition rate. The output of
this laser is frequency doubled to produce 2W average power at 532 nm. This second
harmonic output pumps synchronously two cavity damped dye lasers (Coherent 701-3)
operating with either DCM or LDS698 laser dyes (Exciton). The output of the pump
laser is 100 to 150 mW average power at 8 MHz repetition rate, 5 to 7 ps pulse width,
depending on the dye and wavelength of operation. The probe laser output is ~50 mW

average power at 8 MHz repetition rate, ~5 ps pulse width. For excitation of either of the

49

 

 

4 ~ a

3 _

2 ..
’3
a; 1-
TE
0.) O 1 I 1 I 1 I 1 I 1 1 1 l 1 I 1 I 1
5% 1.78 1.80 1.82 1.84 1.86 1.88 1.90 1.92 1.94
8
.S 4 —
g b
a: 3 1

2 i-

1 ..

1.
1 A 1 I 1 I 1 I 1 I 1 I I I 1 I

 

 

0 1
1.78 1.80 1.82 1.84 1.86 1.88 1.90 1.92 1.94
energy (eV)

Figure 3.1. Linear absorption spectrum obtained from Kramers-Kronig transformation of
the reflection spectrum of a thick crystal of DCHD (z 100nm) in the region of
the polymer backbone excitonic transition at (a) 300 K and (b) 10 K.

50
carbazol side group transitions, the output of the pump laser was frequency doubled with

a Type I L110; second harmonic generation crystal. Average pump power (either red or
UV) at the sample is <1 mW for all of the experiments reported here, and the average
probe power was ~150uW. The pump-probe signal is detected with a radio and audio

frequency triple modulation shot noise limited detection systemml

3.3. Results and Discussion

Crystalline polydiacetylenes exhibit a narrow (~50 meV FWHM), intense excitonic
absorption due to the 7: - 7t* transition localized along the conjugated polymer backbone.
The cross section of this excitonic transition is large; 0t~105 cm'l.m The exact energy of
this resonance depends sensitively on the chemical identity of the side group attached to
the polymer backbone. PTS, a polydiacetylene with p-toluene sulfonate side groups,
exhibits an absorption maximum at 1.98 eV,m whereas DCHD, with N-methyl carbazolyl
side groups, shows an absorption maximum at 1.90 eV.m This difference in the position
of the excitonic absorption maximum is presumed to arise from differences in crystalline
packing forces for these materials, although there has been no direct proof of this
assertion. The excitonic absorption band in crystalline polydiacetylenes exhibits
substantial phonon side structure that persists for several hundred meV above the
excitonic absorption maximum, and these features can be modeled quantitatively based on

[14]

the resonance Raman response of the polymer backbone. The excitonic absorption

manifold is coupled strongly to the ground state vibrations in polydiacetylenes, typical of

51
many organic compounds, and this coupling is a crucial material property for multi-

fiequency optical switching processes. The strength of this coupling depends sensitively
on the extent of structural disorder or any electronic perturbation to the polymer
backbone. Such an electronic perturbation can arise either as a consequence of oxygen

“5] or from coupling to the side groups. It is therefore important to understand

adsorption,
the nature of the coupling between the polymer backbone and its side groups fi'om both a

structural and electronic perspective.

PTS has received the majority of the experimental attention on polydiacetylenes because
of its comparative ease of synthesis and the many earlier works that have served to
characterize its structure and relaxation properties. The ultrafast relaxation processes of
PTS both at room temperature and 10K has been investigatedlm] PTS, and other
polydiacetylenes, exhibit a characteristic optical response on two distinct timescales and
with different spectral signatures. A fast response, which follows the laser cross
correlation (~10 ps), and a slow, energy dependent response, with a characteristic decay
time of hundreds of picoseconds to nanoseconds, is observed subsequent to direct
excitation of the backbone excitonic transition. PTS is considered unique among
polydiacetylenes because the excitonic absorption splits into two resonances separated by
40 meV below 200 K.”6'18] The reason for the observed excitonic splitting is that PTS
undergoes a structural phase transition at 200 K wherein the crystal unit cell doubles and
the p-toluene sulfonate side groups undergo alternate i8° rotations from their high

[19]

temperature conformation. These two side group orientations yield two inequivalent

polymer backbone environments, resulting in two excitonic resonances. It was this

 

 

52
structural anomaly that allowed the identification of the structural origin of the slow

optical response seen in pump-probe experiments on PTS. The slow response exhibited a
spectral profile corresponding to the first derivative of the excitonic transitions, and this
derivative character has proven to be a sensitive indicator of substructure within the
absorption band of polydiacetylenes. A featured slow response has been observed for
DCHD at 300 K and this response was investigated in detail at 10 K. It is likely that the
origin and energy-dependence of the slow response observed for DCHD is fundamentally

the same as that observed in PTS.

The previous chapter discusses the room temperature pump-probe response of DCHD
focused on the ultrafast transfer of excitation from the N-methyl carbazolyl side group to

[8] The N-methyl carbazolyl side group exhibits two allowed

the polymer backbone.
transitions, one along the carbazole long axis (300 nm, 4.13 eV), and the other along the
carbazole short axis (347 nm, 3.57 eV).m The transient response of both excitations
showed a fast bleaching signal followed by a slow, energy dependent recovery. The fast
response follows the laser cross correlation and arises from phase space filling. Earlier
work on PTS has shown that the slow response is due to the modulation of the polymer

backbone by dipolar coupling to vibrational motion of the side groupslm]

The data presented here, taken at 10 K, are shown in Figures 3.2-3.5. The excitonic
transition responds similarly to excitation of either side group transition (3.57 eV, 4.13
eV) or direct excitation of the polymer backbone (1.90 eV). A fast (< 10 ps) bleaching

response followed by a slow energy dependent recovery was observed (Figure 3.2). The

53

/ zero delay time

 

 

 

A 1 I 1 I 1 I

1 1 I
0 100 200 300 400 500

scan time (ps)

Figure 3.2. Time scan of the pump-probe signal for excitation at 1.937 eV and probing at
1.925 eV.

54

 

2 ..
1 #- I- _
I I ./I I .\.‘I-I-I-I -|
O ta 4

 

 

 

 

 

’37 A
5’" J
E .
<1
.4 _
-6 1 .
-3 _. ‘
A I 1 I 1 I 1 I 1 I 1 I
1.82 1.84 1.86 1.88 1.90 1.92 1.94
energy (eV)

Figure 3.3. Transient response of DCHD for excitation at 1.93 7eV direct excitation of the
exciton transition. Fast response (0 ps, I) and slow response (30 ps, 0 ; 125 ps,
A) are shown.

   

55

 

A ' 0
32'2“
3
E .
<
.4_
-6- A

n I A I 1 J j 1 4 J A I

1.82 1.84 1.86 1.88 1.90 1.92 1.94
energy(eV)

 

 

Figure 3.4. Transient responses of the DCHD excitonic transition for excitation at 3.57eV,
the short axis polarized on the carbazole side group. Fast response (0 ps, I) and
slow responses (30 ps, 0; 125 ps, A) are shown.

56

 

 

 

1.0 - /__.,e--\
i- /. I‘-
0.8 - I,. \
/ I
O 6 " I . \I\
h / \ ./\ .\"I
0'4 - ./ \e/ . e\.\l-.
0.2 ~ A /'
_ /\\ 0 \e ‘r‘ l-0
g 0.0 \\ / ‘ / A'— \F—
3 02. ‘ 1/ / \1/
t - ' . '\e/
2 -0.4 ~ ./
-O.6 " A/\
-0.8 - ‘
-10 L \‘
'1.2 l’ ‘
_1.4 b 1 1 1 1 4 A 1 1 1 1 1 1
1.84 1.86 1.88 1.90 1.92 1.94

energy (eV)

Figure 3.5. Transient responses of the DCHD excitonic transition for excitation at 4.13eV,
the long axis polarized on the carbazole side group. Fast response (0 ps, I) and
slow responses (30 ps, O; 125 ps, A) are shown.

57
information contained in both of these responses provides insight into the structural and

energetic couplings in DCHD, and it is the structural information contained in these data

that is the primary focus of this chapter.

One motivation for performing these experiments at low temperature was to attempt to
resolve the fast excitation transfer from the side group to the polymer backbone. The

excited state lifetime of the side group, 11), can, in principle, exhibit a temperature
dependence, and this lifetime appears in the denominator of the expression for the

excitation transfer rate constant in the Forster model‘s]

3K2RéS
ZTDR6

 

[1]

kDA =

If the fast excitation transfer from the side group to the polymer backbone could be
slowed by increasing tn, then the rate could be measured. The data for the fast response
of DCHD at 10 K remain inconclusive in the sense that the transfer of excitation from the
side group to the polymer backbone occurs in <10 ps. Nonetheless, the fast excitation
transfer observed remains consistent with the predictions of the Forster energy transfer

mechanism.

DCHD exhibits a temperature dependent optical response fundamentally difl‘erent than

that of PTS. The linear reflection spectra of DCHD both at room temperature and 10 K

[9-11]

was measured and, using the Kramers-Kronig transformation, converted these data to

58
absorption spectra (Figure 3.1a,b). The 10 K extinction spectrum of DCHD is red shifted

by ~15 meV from that of the room temperature spectrum. Neither phase transition nor
splitting of the absorption peak was observed as is seen for PTS. The observed
substructure within the slow pump-probe response suggests the presence of more subtle
structural heterogeneity in DCHD than has been measured for PTS. The Wegner group
has, through their polymerization studies of polydiacetylenes, shown that the packing of
the side groups determines the crystal structure of the polymer?” Their work also shows
that there is a large void volume in the DCHD crystalline monomer and strain introduced
during polymerization process causes the side groups to reside in a variety of (slightly)
different orientations. The results presented here serve to reinforce this assertion, and the
pump-probe spectroscopic technique used senses the dynamical rather than static

components of this structural heterogeneity.

An energy dependent slow response for all modes of excitation was observed (Figures 3.3-
3.5). The most important aspect of these data is that the spectral profile of this slow
response depends on the excitation energy, and this energy-dependence can be understood
according to the amount of excess energy remaining in the system subsequent to electronic
relaxation. The greater the energy difference between the excitation energy and the
excitonic transition energy, the more features are observed in the slow response. The
distinct features in the slow response are each separated by ~20 meV in DCHD, whereas
the corresponding separation in PTS is ~40 meV."6’m It can be inferred that the

structural variability in DCHD that gives rise to these individual features perturbs the

59
electronic structure of the backbone to a significantly smaller extent than the 418° ring

rotation seen for PTS.

The number of features seen in the slow response are related to the excess spectroscopic
energy imparted to the system on excitation. Excitation of the side groups requires 3.57
or 4.13 eV, depending on the transition accessed. The backbone excitonic resonance is
1.90 eV. Excitation transfer from the side group to the backbone leaves either 1.67 or
2.23 eV of spectroscopic energy to be dissipated amongst the available degrees of
fieedom. Ultimately, the majority of this excess energy must be distributed into the
vibrational modes of both the side groups and the polymer backbone. After excitation of
the side groups and subsequent excitation transfer to the backbone, the side groups are
vibrationally hot. Direct excitation of the backbone imparts no such excess energy in the
side groups. Because dipolar coupling of the side groups to the polymer backbone gives
rise to the slow optical response, the extent to which side group vibrational modes are
populated will determine the number of species that can modulate the backbone
resonance. It is fair to question whether these backbone modulations are side group
vibrational mode-specific for a fixed side group orientation or are related to a distribution
of side group orientations. From measurements on PTS it is known that the individual
features in the slow response correspond to structurally distinct moieties rather than
modulations from multiple vibrational modes. If it is assumed that this relationship holds
for DCHD as well, then the excess vibrational energy remaining in the side groups is
sufficient to distribute the side group among several slightly different orientations. For

direct excitation of the polymer backbone, one resonance dominates the slow response

60
(Figure 3.3). For excitation at 3.57 eV, a different and broader resonance is seen, with at

least two others discernible. Excitation at 4.13 eV yields four resonances in the slow
response. Because it is unknown how the individual electronic states of the N-methyl
carbazole moiety couple to the available vibrational degrees of freedom, i. e. ground state
side group vibrational modes, it is not possible to predict how the excess vibrational
energy will be partitioned, especially when the time scale of the observation is similar to
the expected vibrational population relaxation times of organic moleculesml
Measurement of the resonance Raman excitation profiles of both side group transitions

1 However, the

would provide the information necessary to resolve this question.‘14
amount of excess energy remaining in the system could be correlated with the extent of
observed structural heterogeneity. If it is assumed that the excess energy is distributed
into all modes uniformly, the temperature rise in the DCHD crystal resulting from
excitation of the side groups could be estimated. Assuming the thickness of the crystal to
be 1000A the diameter of the beam spot at the crystal surface to be 25pm, and the
excitation pulse to be functionally instantaneous, excess energies of 2.23 eV and 1.67 eV
would change the temperature of the sample by ~9K and 7K, respectively, for these
experimental conditions. These temperature increases are at least qualitatively consistent
with the results obtained, but the differences in the slow responses for 1.67 eV and 2.23

eV excess energy (Figures 3.4 and 3.5, respectively) suggest different pathways for

disposal of the excess energy from the two side group excited electronic states.

6 1
3.4. Conclusion

The pump-probe spectroscopic data on DCHD at 10 K show the rate of excitation transfer
from the side group to the polymer backbone remains unresolvably fast (< 10 ps). While
this result is consistent with the predictions of a Forster excitation transfer mechanism, it is
not possible from these data to evaluate the rate constant for this process. The energy
dependent slow response at 10 K indicates the presence of several subtle conformers
within the exciton envelope, and suggests that the energetic barriers between these

conformers is < 0.5 kcal/mol.

62
Literature Cited

fl

. D. S. Chemla and Z. Zyss, eds, Nonlinear Optical Properties of Organic Molecules
and Crystals, Vol. 2, Academic Press, New York, 1987.

N

. G. J. Blanchard, J. P. Heritage, A. C. Von Lehmen, M. K. Kelly, G. L. Baker and S.
Etemad, Phys. Rev. Letters, 63, 887(1989).

DJ

. G. J. Blanchard and J. P. Heritage, J. Chem. Phys., 93, 4377(1990).
4. M. E. Morrow and C. J. Eckhardt, Chem. Phys. Letters, 144, 65( 1988).

5. L. Sebastian and G. Weiser, Phys. Rev. Letters, 46, 1156(1981).

O5

. L. Sebatian and G. Weiser, Chem Phys., 62, 447(1981).

\)

. R. J. Hood, H. Muller, C. J. Eckhardt, R. R. Chance, and K. C. Yee, Chem. Phys.
Letters, 54, 295( 1978).

00

. S. A. Hambir, T. Yang, G. J. Blanchard and G. L. Baker, Chem. Phys. Letters, 201,
521(1993).

9. T. S. Robinson, Proc. Phys. Soc. (London), B65, 910(1952).

10. T. S. Robinson and W. C. Price, Proc. Phys. Soc. (London), B66, 969(1953).
11. G. A. Anderson and W. B. Person, J. Chem Phys., 36, 62(1962).

12. Y. Jiang, S. A. Hambir and G. J. Blanchard, Opt. Commun, 99, 216(1993).

13. G. J. Blanchard and M. J. Wirth, Anal. Chem, 58, 532(1986); P. Bado, S. B. Wilson
and K. R. Wilson, Rev Sci. Instrum, 53, 706(1982); L. Andor, A. Lorincz, J.
Siemion, D. D. Smith and S. A. Rice, Rev. Sci. Instrum, 55, 64(1984).

14. D. N. Batchelder and D. Bloor, J. Phys. C, 15, 3005(1982).
15. G. J. Blanchard and J. P. Heritage, Chem. Phys. Letters, 177, 287(1991).

16. M. J. Nowak, G. J. Blanchard, G. L. Baker, S. Etemad, Z. G. Soos, Phys. Rev, B41,
7933( 1990).

17. D. Bloor and F. H. Preston, Phys. Stat. Solidi (a), 39, 607(1977).
18. M. Schott, F. Batallan and M. Bertaykt, Chem Phys. Letters, 53, 443(1978).

19. V. Enkelmann and G. Wegner, Maldomo]. Chem, 178, 635(1977); V. Enkelmann,
Acta. Cryst, B33, 2842(1977).

63
20. V. Enkelman, R. J. Leyrer, G. Schleier, G. Wegner, J. Mat. Sci, 15, 168(1980).

21. S. A. Hambir, Y. Jiang and G. J. Blanchard, J. Chem. Phys., 98, 6075(1993).

CHAPTER 4

Disorder Induced Enhancement of the Third Order

Optical Nonlinearity in a Conjugated Polymer

Abstract

The relationship between material disorder and nonlinear optical response for a conjugated
polymer was investigated. Inverse Raman scattering, the technique used here, is sensitive
to the transition cross sections of both the purely electronic (81"=0 <—-) Sovzo) and vibronic
(S {=0 (-) So‘F‘) transitions accessed, and both of these quantities are related to the extent
of disorder in the polymer backbone. These morphology dependencies were verified by
using crystalline and amorphous samples of the polydiacetylene poly(4BCMU). The
magnitude of the nonlinear response per backbone repeat unit in the amorphous material is
comparable to that of the crystalline form because of the large potential well displacement
between the ground and excited electronic states characteristic of the disordered material.
This increased potential well displacement compensates for the nonlinear response lost to
the reduced macroscopic polymer alignment associated with the 2-dimensionally isotropic

spin cast film.

64

65
4.1. Introduction

The routing and storage of electronic signals is the fundamental action by which
computing and related technologies operate. The properties intrinsic to semiconductor
materials limit how fast electronic signals may be processed, and improvements beyond
this point will require either new materials, new signal processing strategies, or a
combination of the two. Replacing electrons with photons is a logical step in achieving
faster information processing. Unfortunately, the technology and methodology for
processing photonic signals, without first converting them into electronic signals, lags far
behind the ability to send them over long distances. Advancing the field of photonic signal
processing will require significant progress in both materials and switching strategies. If
photon-based switching is to bear any resemblance to the architecture used for present-day
electronic switching, which it must to ensure backward-compatibility, then materials that
possess a large nonlinear optical response will be the cornerstone of this new technology.
An all-optical switching material must change state or “turn on” rapidly without being
excited directly, because the lifetime of a resulting excited state would limit its “turn off’
time. A large nonresonant optical nonlinearity is therefore needed, and polydiacetylenes,
because of their extensive conjugation, possess among the largest nonresonant third order
optical nonlinearities known, x”) ~ 10'9 esu.“’2] Despite this favorable optical property,
most polydiacetylenes are available only as crystalline materials. While ideal for
fundamental spectroscopic investigations, this structural property limits the potential utility
of these materials for device applications. One family of polydiacetylenes, however,

possesses urethane based side groups, and the labile nature of these side groups allows the

66
polymers to be synthesized and processed into a range of morphologies ranging from

single crystals to amorphous films. The focus of this work is on determining the
morphology-dependence of the nonlinear response for poly(4BCMU), a widely studied

processable polydiacetylene (Figure 4.1).

Most device-oriented work aimed at exploiting x“) has focused on conjugated polymers in
waveguide formats because of the compatibility of this device structure with
semiconductor electronics and optical fiber technology.[3'“] The preparation of
waveguides and linear waveguiding have been demonstrated for both crystalline and
amorphous thin films of polydiacetylenes. Nonlinear effects have been characterized as
well, but the unambiguous demonstration of all-optical switching in waveguide devices has
been elusivelm” The fundamental limitation of such devices is the deleterious efiects
that both linear and nonlinear absorption have on waveguide performance. For example,
linear losses not only reduce the intensity of the signal in the waveguide, but also lead to a
slow, thennally-induced refiactive index change that often overwhelms the nonlinear
response and severely limits the duty cycle attainable for all-optical switching.[3'”] A
further complication is that the nonlinear susceptibility contains both real and imaginary
terms, and only the real part of the response (n2) is useful for waveguide-based optical
switching. The most significant contribution to the imaginary part of 1(3) for this type of
device is two-photon absorption. For crystalline polydiacetylenes waveguide-based
switching is limited to wavelengths 2 1.3um with the low energy limit being determined by

linear losses from overtone absorptions in the near-[Rm Switching strategies based on

conjugated polymers in waveguide formats will be difficult to implement, and will be

67

Ii I1 I93" I3
MO’CVNC’OMMO’CKVE’OW

 

0 1'4 II o”

O H” H O”
I l ("3 O
Mdévi‘kgo \/\/\o/ \IIqAC/ W
H

 

Figure 4.1. Poly[5,7-dodecadiyne-1,12-diolsbis(n-butoxycarbonymethylurethane)] or
Poly(4BCMU). The dashed lines indicates hydrogen bonding between the side

groups.

68
impractical in the 0.7-1.3 um wavelength range where many high powered solid state

lasers are available. If these materials are to be usefirl for photonic signal processing

applications, then alternative switching strategies will have to be employed.

Blanchard et. a]. have reported on the inverse Raman scattering response of the

polydiacetylene PTs,ilé-181

a crystalline polydiacetylene (with rigid p-toluene sulfonate
sidegroups) that possesses the same large nonlinear optical response as poly(4BCMU).
The motivation for investigating poly(4BCMU) using inverse Raman scattering is to
determine the effect(s) of structural disorder within the nonlinear chromophore on its
optical response. This question is central to determining the ultimate utility of these
materials for applications as photonic logic gates. The focus here is on the fundamental
relationship between polymer morphology and nonlinear optical response in this material.
The inverse Raman response of polydiacetylenes can be modeled quantitatively as a
strongly coupled three level system, and the transition cross sections for the transitions
accessed depend on the morphology of the polymer. The data presented here
demonstrate that the deterministic transition cross section for the inverse Raman process is
larger for the disordered material than for the corresponding crystalline material. This is
an expected result based on the shorter effective conjugation length characteristic of the

disordered material, and the reasons for this enhancement in the nonlinear response of

disordered poly(4BCMU) are detailed below.

69

4.2. Theory

In order to provide a frame of reference for the motivations behind this work, the sort of
optical device for which these materials will be usefirl must be considered. While there are
many possible optical switching schemes, perhaps one of the most conceptually straight-
forward is that of an ultrafast optically-triggered shutter. This shutter could operate in
either of two modes; normally open or normally closed. This work will concentrate on the
latter because, for such a device, high contrast is not necessary; the switched signal exists
against a dark background. For this device, a “carrier” light pulse incident on the material
would be in resonance with the (excitonic) electronic transition and is either absorbed or
reflected except when a “trigger” light pulse is coincident with the carrier pulse. The
trigger pulse perturbs the excitonic absorption band in such a way as to allow the
transmission of the carrier pulse. Only when both the trigger and carrier pulses are present
does the material transmit at the carrier pulse frequency. Such a device is the optical
analogue of a digital AND gate. The mechanism of the nonlinear response used as a

"9] one of a family of

“shutter” effect is stimulated inverse Raman scattering,
nondegenerate four wave mixing processes. A significant reason for using nondegenerate,
instead of degenerate, wavernixing spectroscopy is that, for nondegenerate processes, the
detected optical response depends critically on the nuclear, i. e. vibrational, as well as

electronic response of the system. For a coupled three level system (Figure 4.2), the third

order susceptibility describing inverse Raman scattering is given by”

70

 

 

 

 

 

X A
(Dvx 030X
V
A
(Dov
O V

 

Figure 4.2. Schematic three level system.

71
lfloxlzlflvxl2 l
’73 (€001 ' ‘01 470920001: — ‘0: + wp ‘ ii’ov)

 

2(3)(w1)= [1]

where the terms u are dipole moments for the appropriate transitions, 03's are frequencies
and 7's are linewidths. The subscripts refer to the levels shown in Figure 4.2, “t” is for
“test” (= carrier) and “p” is for “pump” (= trigger), where the test and pump electric fields
are provided by two independently tunable lasers. The important features in the inverse

[16,20-22]

Raman response, including the phonon-mediated optical Stark effect, arise from

interference between the two complex terms in the denominator of Equation 1.

The relationship between polymer morphology and inverse Raman response is
straightforward and is determined largely by the numerator terms in Equation 1. 1(3) is
related to the square of the dipole moments for both the purely electronic X (——> 0 and the

vibronic X (—-) v transitions. The square of the transition dipole moment is related to the

transition cross-sectiomml

_ 8n3waab

3’10 IJ Vapab Wbdtlz [2]

Job

where a and b are the two states involved in the transition. x“) is proportional to the

transition cross-sections and therefore the overlap integrals for those transitions, and the

72

1(3) 09 001011 It <0|r)2<le)2 [3]

overlap integral for a transition is a function of the displacement between the ground and
excited electronic state potential wells. The potential well displacement can be controlled
by macroscopic polymer processing techniques. For identical ground and excited state

harmonic oscillators, the displacement is expressed in terms of the dimensionless quantity

2 [24]

3

1 =(212fljyzp [41

where m is the effective mass of the system, f is the vibrational frequency and D is the well

displacement. Blanchard et. al. have shown that polydiacetylenes can be treated to

1 [17,18]

excellent approximation using this harmonic oscillator mode The harmonic

(3)

oscillator overlap integrals and resultant x are (for a v = 1 mode)[241

(0|0) = exp(_ 2%) (1|0) = -2 exp(- 232/2)

[5]
1(3) oc z2 exp(— 222)

73

‘3’ on the potential well displacement is shown in Figure 4.3. For the

The dependence of x
polydiacetylene PTS the largest value of z, for the backbone C=C stretch, is ~0.5.“7'2“l
The magnitude of the two-color nonlinear response of a polydiacetylene can be increased

by almost a factor of two if D can be optimized. The experimental data, described below,

demonstrate that inducing disorder in poly(4BCMU) can serve to optimize D.

4.3. Experimental

4.3.1. Laser Spectroscopy. Stimulated inverse Raman spectroscopy is performed using a
pump-probe laser spectrometerm'ls’m For these measurements an intense “pump” pulse
provides sub-resonant excitation and a weak “test” pulse approximately one vibrational
resonance higher in energy than the pump pulse interrogates the result of the excitation.
The spectrometer used to generate these pulses has been described in detail in chapter 1,
and only a brief synopsis is provided in here. The source laser is a mode-locked CW
Nd:YAG laser (Coherent Antares 76-S) that produces 3W (average) at the second
harmonic wavelength, 532 nm, and 1W (average) at the third harmonic, 355 nm. The
repetition rate of this laser is ~76 MHz and the pulsewidth is ~100 ps FWI-M. The output
from this laser is used to excite two cavity dumped dye lasers (Coherent 701-3). The
pulsewidths output from these lasers are 5 ps FWHM and the cross correlation between
the two dye lasers is ~10 ps FWHM. For experiments on the crystalline polymer, the
pump laser wavelength was varied between 730 nm and 680 nm (LDS 698 dye, Exciton)
and the probe (test) laser tuned over the range of 670 nm to 635 nm (Kiton Red,

Exciton). For experiments on the spin cast polymer film, the pump laser wavelengths

0.20

0.15

susceptrbility (an)

0.05

0.00

0.0

74

I

 

\e“

0.5 2.0

1.0 1.5

Figure 4.3. Nonlinear susceptibility as a fimction of potential well displacement.

75
were set between 627 nm and 579 nm (Rhodamine 6G, Rhodamine 610, Exciton) and

probe laser wavelengths were tuned over the range of 565 nm to 520 nm (pyromethene
567 and coumarin 500, Exciton). The pump-probe signals are detected using radio- and
audio-frequency triple modulation signal encoding with synchronous demodulation
detection, allowing shot noise limited sensitivity and at least 4 orders of magnitude of

[26-28]

linear dynamic range. This wide dynamic range is necessary for the quantitative

measurement of the subtle features present in inverse Raman data.

Experimental stimulated inverse Raman spectra are obtained for a particular pump energy
through a series of time scans at different probe frequencies. Though tedious, this
technique measures signal intensities quantitatively because of its combined high time and
fiequency resolution. Families of inverse Raman spectra, taken at a progression of pump

energies, are fit to calculated spectra to obtain ovx.

4.3.2. Materials processing. The synthesis of poly(4BCMU) can yield either the
crystalline (blue phase) form of the material or, subsequent to polymerization, can be
dissolved in an appropriate solvent and spin coated onto rigid substrates. For the
preparation of the spin cast film, the polymer was mixed 5% by weight with
cyclopentanone and dissolved by heating at 35°C and stirring. The resulting solution was
spin coated at 1000 rpm for 60 seconds onto a quartz substrate. These two forms of

poly(4BCMU) represent the extrema in the extent of disorder attainable in this material.

76

4.4. Results and Discussion

The magnitude of x“) in polydiacetylenes is related to the oscillator strength of the exciton
that dominates the absorption edge, and optical pumping schemes that couple vibrational

)“6’21’221 can be used

modes to the exciton (e.g. the phonon mediated dynamic Stark effects
to effect a nonlinear switching response that is not hindered by linear losses. These
dynamic Stark effects have been measured and modeled quantitatively in crystalline forms
of polydiacetylenesm'm The exciton-phonon coupling that detemrines the efficiency of
these Stark effects is intimately related to polymer ordering through the exciton line shape
(7,. in Equation 1) and potential well displacement, Dim Poly(4BCMU) is the most

[2930] whose

widely studied member of a family of poly(nBCMU) polydiacetylenes
macroscopic crystallinity is determined by the hydrogen bonded network formed by the
urethane-based side groups (indicated by dashed lines in Figure 4.1). Disruption of this
hydrogen bonded network reduces local order, interrupting the conjugation of the polymer
backbone, thereby causing a dramatic change in the linewidth and energy of the excitonic
response?” The position of the excitonic transition is remarkably sensitive to changes in
polymer morphology, a ~3 00 meV blue shift and broadening of the excitonic resonance is
observed on solvent disordering of crystalline poly(4BCMU) (Figure 4.4). For crystalline
poly(4BCMU) the excitonic transition occurs at 1.96 eV and for the spin cast film, the

transition is found to be centered at 2.29 eV. Both the shift and the transition line width

can be used to characterize disorder in these materials. Poly(4BCMU) is processable into

77

 

 

 

 

crystalline poly(4BCMU)
\ spin-cast poly(4BCMU) film
0.8 *-
0.6 -
8
5
-E
8
'2 0.4 —
0.2 -
0.0 1 I 1 I 1 L 1 4 1 I 1 I 1 I 1 I 1
1.6 1.8 2.0 2.2 2.4 2.6 2.8 3.0 3.2
Energy, cV

Figure 4.4. Linear absorption spectra of crystalline and spin cast film forms of
poly(4BCMU). For the crystalline polymer, 0)ox = 1.96 eV and for the spin cast
fihn, c0... = 2.29 eV.

78
morphologies ranging from single crystals to films that are isotropic in two

dimensionsn’m

To understand the relationship between polymer disorder and the excitonic transition
energy and linewidth, it is useful to consider the optical response of polydiacetylenes in the
context of a one-dimensional particle in a box. For such a system the energy level spacing,

AE, depends on the inverse of the square of the length of the “box”, L,

h2

AE=——2
8mL

[6]

Equation 6 implies that, for an infinite polymer chain, the energy gap should tend to zero.
This clearly does not correspond to experimental data, and the practical limit to this
interpretation lies in the characteristic length of the exciton, which is determined by the
coulombic and correlation forces intrinsic to the conjugated backbone 7: system. If the
spacing between interruptions in conjugation along the polymer backbone is greater than
the characteristic exciton length plus the distance the exciton can migrate during its ~2 ps
lifetime?” then the chain appears “infinite” to the excitation. Introducing disorder into
the polymer reduces the effective length of the conjugated segment, thereby increasing
AB. The presence of disorder also implies a distribution of conjugation lengths because

the location of defects along the backbone is necessarily statistical. A distribution of

conjugation lengths will result in a distribution of transition energies, A(AE). The

79
introduction of disorder in polydiacetylenes has two measurable effects on the linear

optical response of these materials - a spectral shift and an increase in the measured

linewidth of the excitonic resonance.

Figures 4.5 through 4.8 show inverse Raman optical responses of both crystalline and spin
cast poly(4BCMU). The intrinsic nonlinear response that gives rise to the inverse Raman
spectra can be modeled qualitatively using Equation 1 in the small signal limit. The
Schmitt-Rink modelm] used in this calculation is more complete, and its application to
experimental data has been described in the paper by Blanchard etalm'm The signal
detected experimentally is related to Imam} through the complex dielectric response of

the polymer. In the limit of a small signal and a thick sample,
—- z Aat = (——) Ak [7]

Ak = k(pump on) - k(pump off), i is the material thickness, 0) is the probe laser frequency
and or is the material absorptivity at frequency 0). Ak is related to )6” through the complex

dielectric response of the material.

1
Air = [(21812 + A822)y2 - Asl]/2
A191 2 1+ 47rRe{ 1(3)} [3]

A52 = 47tIm{ 1(3)}

80

0 06 :- a) wpump = 1.774 CV

 

 

 

 

 

 

o
o
o

W J;

1 I 1 I 1
1.90 1.95 2.00 2.05 2.10
0) eV

probe,
0.006 f b) comm: 1.741 eV
0.004 -
0.002 -
E2 0.000 r
-0.002 -
-0004 1
-0.006 r
'0'008 h 1 1 1 1 1 1 1

1.85 1.90 1.95 2.00 2.05

00 , eV
probe

/1‘

 

 

 

 

 

 

Figure 4.5. Experimental data points and calculated fit (solid line) for a) stimulated inverse
Raman spectrum of crystalline poly(4BCMU) for to, = 1.774eV and b) for (0, =
1.741eV.

81

0.004
0.002 p
0.000 _
-0002 L
-0004 L

'0-006 ' 1 1 1 i 1 1 1 1 1
1.80 1.85 1.90 1.95 2.00

0) , eV
probe

 

 

AT/T

 

 

0.0015 - b)(1) = 1.653 eV
0.0010 '-
00005 5 -,_
0.0000 PM“ '-
00005
00010
-00015 P

-0.0020'- I .1 .1. I
1.75 1.80 1.85 1.90 1.95

0) eV

probe,

V I I

AT/T

I l T l T

 

 

Figure 4.6. Experimental data points and calculated fit (solid line) for a) stimulated inverse
Raman spectrum of crystalline poly(4BCMU) for (0,, = 1.701eV and b) for 00,, =
1.653eV.

82

 

 

 

 

0.003 _ a) copump = 2.141 eV
0.002
5 0.001 M
< '- ".eel ,e-‘ ..
0.000 _ PW
0001 ‘3’
-0.002 ‘ ' ‘ 1 4 ' ‘ '
2.25 2.30 2.35 2.40 2.45
0) , eV
probe
0003 F b) mpump = 2.113 CV
0.002
I: .
E3 0.001

 

0.000

 

 

 

_0.0011111111114111

2.28 2.30 2.32 2.34 2.36 2.38 2.40

00 (eV)
probe

Figure 4.7. Experimental data points and calculated fit (solid line) for a) stimulated inverse
Raman spectrum of spin cast poly(4BCMU) for top = 2.141eV and b) for 00p =
2.1 13eV.

83

0,003 ..a) 0) =2.083 eV

 

 

 

 

 

 

 

 

 

 

 

_ PumP
0.002 r-
1: 0.001 -
E“ .
4
0.000
-0.001 -
{1002 b 1 1 1 1 1 1 1 1
2.20 2.25 2.30 2.35 2.40
wprobe’ 6V
b) 6 =1.978 eV
In pump "\
0.005 -
t i-
f; 0.000
-0.005 -
1 1 1 1 1 1 1 1
2.10 2.15 2.20 2.25 2.30
0) , eV
probe

Figure 4.8. Experimental data points and calculated fit (solid line) for a) stimulated inverse
Raman spectrum of spin cast poly(4BCMU) for 0),, = 2.083 eV and b) for 0),, =
1.978eV.

84
For materials with a large energy-dependent dielectric response, such as conjugated

polymers, the difference between AT/T and Imam} can be significant. Using the
relationships shown in Equations 8, virtually quantitative agreement between theory and
experiment for both crystalline and spin-cast poly(4BCMU) has been achieved. Any slight
difl‘erences between experiment and calculation can be traced to the use of a Lorentzian
lineshape function in the theory and the experimental deviations from this lineshape

assumption.

Figures 4.5-4.8 contain both the calculated and experimental inverse Raman spectra for
crystalline and spin cast poly(4BCMU). The signal size increases as the overlap between
the vibrational resonance and the electronic resonance is increased. As can be seen from
the linear responses for the two forms of the polymer (Figure 4.4), the linewidth of the
electronic transition changes significantly with disorder. What is not apparent from the
linear responses, however, is that the effective linewidth of the exciton in the spin cast film
is similar to that for the crystalline polymer, (Yx ~ 50 meV) with the vibrational linewidth
for the spin cast films being significantly wider than for the crystals (yvf‘lm ~ 9 meV). This
finding indicates that, while the shift in the excitonic transition energy is due to change(s)
in the conjugation length, the majority of the spectral broadening in the film arises from
the distribution of conjugation lengths present in the film, as discussed above. This finding
also implies that the cross section for the dominant excitonic transition, cox, in the film is
diminished to only a small extent on disordering. This point is discussed below. For the
calculations reported in Figures 4.5-4.8, the linewidth ratio (Vex/70v) = 17 and 5.5 for

crystalline and spin cast polymers, respectively, were found to be the best fits to the

85
experimental data. The value of the linewidth ratio affects the size and shape of the

inverse Raman response. For off resonance conditions, where 0),, + 0),," >> 000,, and 0), >>
0),,, the Raman response is primarily absorptive and the sign of AT/T changes from
positive to negative as 0). increases through 0),,, This sign change at the excitonic
resonance is a blue-shift of the electronic transition caused by both the electronic and
phonon-mediated optical Stark effects. The contribution to the AT/T response from these
Stark effects increases as the linewidth ratio decreases. On resonance, where the pump

energy is exactly fromv lower in energy than the electronic resonance and at or = 0).... a

transmissive (positive ATfI‘) response with absorptive side bands is observed. These
features are the result of splitting of the excitonic transition due to the resonant phonon-
mediated optical Stark effect. The “hole” centered at 0),,x is larger than the absorptive
inverse Raman response, and a transmissive feature is observed. The spectral width of the
hole is detemrined by the electronic and vibrational linewidths and the Rabi frequency, R =

popr. The details of this transmissive feature are sensitive to higher order nonlinearities, x

(5), x"), etc?” but these effects will be negligible for realistic experimental conditions.
Between these two extremes, the inverse Raman response appears dispersive due to
interference between the vibrational and electronic resonances, i. e. the denominator terms

in Equation 1.

The experimental data show that the nonlinear optical response from unaligned spin cast
films compares favorably with the response from crystalline poly(4BCMU). The primary

difference between the nonlinear response of a crystal and a spin-cast film of

86
poly(4BCMU) lies in the absolute size of the response. The qualitative nature of the

signal is largely unaffected by modest changes in line widths and variations in the linear
dielectric response, although the quantitative details of the lineshapes and intensities
depend critically on the cross sections and linewidths. Indeed, it is this quantitative
information that is important in elucidating the role that polymer disorder plays in affecting
the nonlinear response. For a spin cast film, there are two competing factors that serve to
approximately cancel one another. A reduction in the intensity of the nonlinear response is
expected solely because of inefficient alignment of the nonlinear chromophore(s) with the
incident electric field. Compensating for this reduction is an increase in the magnitude of
the nonlinear response per chromophore due to a reduction in effective conjugation length
of the nonlinear chromophore. By fitting the experimental data to the Schmitt-Rink
model, and using dielectric corrections, the transition cross sections, 6.x, for the C=C and
Csc stretching resonances in both crystals and films were obtained (Table 4.1). In these
fits to the data, it was determined that the optimum sample thickness and transition cross
section consistent with all of the spectra taken for a given type of sample, either crystalline
or spin cast film. For a given set of calculations there are no adjustable parameters. The
sample thickness (1 = 700 A for crystalline poly(4BCMU) and I = 300 A for the spin cast
film) and transition cross sections (Table 4.1) were held constant for all calculations of the
spin cast film spectra, and a different set of optimized values were used for the calculation
of crystalline polymer spectra. The way to determine the optimum values of o and I has
been described by Blanchard et. aim“) and we will not detail this procedure here. From

these fits to the data the cross sections va for the double and triple bond resonances were

87
determined and are almost a factor of two larger for the spin cast film than for the

crystalline polymer.

Table 4.1. Experimental v-X transition cross-sections and maximum AT/T values for
backbone double and triple bond stretching resonances in crystalline and spin

 

 

cast films of poly(4BCMU).
C=C stretch CsC stretch
va (eV) (AT/T)”, ovx (eV) (AT/T)“,
crystal 5 0.15 0.014 0.07 0.0023
spin cast film 0.26 0.0021 0.15 0.0019

This is an expected result based on the information presented in the theory section. The
dependence of the nonlinear response on polymer disorder can be understood at an
intuitive level. Both the ground state and excitonic equilibrium internuclear distances, re,
depend on the 7t electron density of the conjugated system over the exciton coordinate.
On excitation, the electron density of the 7: system is reduced by one electron, and hence,
the strength of the 7: bonds decreases, making r1. larger than r1. This reduction in electron
density is distributed over the effective conjugation length of the system. The longer the
conjugation length, the smaller the change in electron density per bond on excitation.
Thus a disordered material, having a displacement larger than that for the corresponding
crystalline material partially compensates for the loss of nonlinear response associated with

a processing-induced loss of anisotropy. These experimental data demonstrate that va for

the C=C and CaC backbone stretching resonances are almost a factor of two larger for the

88
spin cast film than for the crystalline polymer. This model predicts that with the increase

in va there will be a corresponding decrease in cox. Because of the functionality of the
overlap integrals, however, (see Equations 5) an increase in potential well displacement
results in an increase in x“).

The data presented in Figures 4.5-4.8 also provide structural information analogous to
that available from spontaneous resonance Raman measurements. The frequencies of the
C=C and CsC backbone stretching resonances in poly(4BCMU) are sensitive to disorder
in the system. Zheng et all?“ have found that the spontaneous resonance Raman response
of spin cast films of poly(4BCMU) exhibits a significant excitation energy dependence.
Specifically, the energies of both the C=C and C2C vary according to the excitation
energy, and the variation does not depend in a simple way on the excitation frequency

(Table 4.2). Zheng eta]. concluded from this work that different excitation wavelengths

Table 4.2. Experimental frequencies of C=C and Csc stretching resonances for spin cast
film poly(4BCMU) as a function of excitation energy.

 

 

C=C stretch CsC stretch
911w) .1. (eV) .1. (eV)
2.141 0.173 0.254
2.113 0.184 0.258
2.083 0.184 0.258
1.978 0.190 0.256

 

selected regions with a characteristic extent of disorder, or a particular conjugation length.

Different excitation wavelengths selected different subsections of the cast film, and

89

because the C=C and CEC resonances are isolated to the polymer backbone, it is not
surprising that their center frequencies depend critically on the extent of backbone order in
the system. The inverse Raman data on poly(4BCMU) films also exhibit an excitation
energy dependence, and we believe the origin of this effect is the same as that for the

spontaneous resonance Raman data.

As mentioned earlier, while the nonlinear response of spin cast films of poly(4BCMU) is
enhanced per backbone nonlinear chromophore unit compared to the crystalline material,
the magnitude of the signal (for the same sample thickness) is close to the same for the
two systems due to the orientational disorder intrinsic to the spin cast film. Disorder
induced enhancement of x“) in poly(4BCMU) can be increased filrther by controlling the
macroscopic backbone orientation within the spin cast polymer film. The backbone
orientation of the polymer can be controlled by stretch alignment. This processing
technique optimizes the long range orientation of the polymer while maintaining the short
range disorder that serves to enhance )6”. For thin polymer films, interactions between
the poly(4BCMU) film and the substrate can also influence the nonlinear response of the

system. These processing-related effects are described in the next chapterm

90
4.5. Conclusion

Stimulated inverse Raman scattering has been used to examine the dependence of 1‘3 ) on
material disorder. The data on crystalline and spin cast films of poly(4BCMU) show that
little if any effective nonlinear response is lost in the amorphous material (Table 4.1). The
enhancement of nonlinear response in a disordered polymer can be accounted for by the
large displacement between the ground and excited state equilibrium distance which
partially restores the nonlinear response. The magnitude of the AT/T signal is modest in
the experiments reported in here because the measurements were made in the small signal
limit, but the observed transient change in transmission of ~1% for the ~10 11] pulse
energies used indicates that this method of accessing x‘” is extremely efficient and that,
ultimately, this optical signal processing strategy will prove successful for photonic logic
device applications. These data also demonstrate that there is a significant opportunity to
enhance the )6” response in processed films compared to the corresponding crystalline
polymer because stretch alignment will return a portion of )6” lost to macroscopic

alignment while maintaining the enhancement in x“) arising from microscopic disorder.

F”.

91
Literature Cited

fl

. C. Sauteret. J. P. Hermann, R. Frey, F. pradere, J. Ducuing, R. H. Baughman and R. R.
Chance, Phys. Rev. Lett., 36, 956(1976).

N

. P. W. Smith, Bell System Tech. Journal, 61, 1975(1982).

Le)

. G. I. Stegeman, A. Miller in Photonics and Switching, Ed. J. E. Midwinter, Academic
Press, London, 1994.

4. G. L. Baker in Contemporary Topics in Polymer Science, Vol. 7, Ed. by J. C. Salamone
and J. Riffle, Plenum Press, New York, 1992, pp. 269-277.

LII

. G. I Stegeman, E. M. Wright, Opt Quant. Elect, 22, 95( 1990).

ON

. S. R. Friberg, Y. Silberberg, M. K. Oliver, M. J. Andrejco, M. A. Saifi and P. W.
Smith, Appl. Phys. Lett., 51, 1135(1987).

7. P. D. Townsend, G. L. Baker, N. E. Schlotter, C. F. Klausner and S. Etemad, Appl.
Phys. Lett., 53, 1782( 1988).

on

. P. D. Townsend, G. L. Baker, N. E. Schlotter, C. F. Klausner and S. Etemad, Synth.
Met, 28, D633(1989).

9. W. Krug, E. Miao, M. Derstine and J. Valera, J. Opt. Sco. Am. B, 6, 726(1989).
10. T. Kaino, K. Jinguji and S. Nara, App]. Phys. Lett., 41, 802(1982)

11. N. E. Schlotter, J. L. Jackel, P. D. Townsend and G. L. Baker, Appl. Phys. Lett., 56,
13( 1990).

12. D. M. Krol and M. Thakur, App]. Phys. Lett., 56, 1406(1990).

13. M. C. Gabriel, N. A. Whitaker, C. W. Dirk, M. G. Kuzyk and M. Thakur, Opt. Lett.,
16, 1334(1991).

14. P. D. Townsend, J. L. Jackel, G. L. Baker, J. A. Shelbume, HI and S. Etemad, Appl.
Phys. Lett., 55, 1829(1989).

15. A. Tanaka, H. Sawada, T. Takoshima and N. Wakatsuki, Fiber Integ. Opt, 7,
139(1988).

16. G. J. Blanchard, J. P. Heritage, A. C. Von Lehmen, M. K. Kelly, G. L Baker and S.
Etemad, Phys. Rev. Lett., 63, 887(1989).

17. G. J. Blanchard and J. P. Heritage, J. Chem. Phys., 93, 4377(1990).

92
18. G. J. Blanchard and J. P. Heritage, Chem. Phys. Lett., 177, 287(1991).

19. W. J. Jones and B. P. Stoicheff, Phys. Rev. Lett., 13, 657(1964).
20. S. Saikan, N. Hashimoto, T. Kushida and K. Namba, J. Chem. Phys., 82, 5409(1985).
21. M. Yoshiuzawa, Y. Hattori and T. Kobayashi, Phys. Rev. B, 47, 3882(1993).

22. T. Kobayashi, M. Yoshizawa, M. Hasegawa and M. Taiji, J. Opt. Soc Am B, 7,
1558(1990).

23. G. Herzberg, Molecular Spectra and Molecular Structure, Vol. 1, Van Nostrand,
New York, 1950, p.383.

24. D. N. Batchelder and D. Bloor, J. Phys. C, 15, 3005(1982).

25. Y. Jiang, S. A. Hambir and G. J. Blanchard, Opt Commun, 99, 216(1993).
26. G. J. Blanchard and M. J. Wirth, Anal. Chem, 58, 532(1986).

27. P. Bado, S. B. Wilson and K. R. Wilson, Rev. Sci. Intrum, 53, 707(1982).

28. L. Andor, A. Lorincz, J. Siemion, D. D. Smith and S. A. Rice, Rev. Sci. Instrum, 55,
64(1984).

2

\O

. T. Kobayashi, J. Lumin, 58,1 17(1994).

30. S. Molyneux, A. K. Kar, B. S. Wherrett, T. L. Axon and D. Bloor, Opt Lett., 18,
2093(1993).

31. M. J. Nowak, G. J. Blanchard, G. L. Baker, S. Etemad and Z. G. Soos, “Slow
Relaxations in PDA-4BCMU: From Crystals to Films”, in Conjugated Polymeric
Materials: Opportunities in Electronics, Opto-Electronics and Molecular
Electronics, Ed. by J. L. Breda and R. R. Chance, NATO ASI Series B, Vol. 182,
Kluwer Academic Publishers, Dordrecht, 1990, 421-427.

32. G. N. Patel, Polym Prepr., ACS Div. Polym. Chem, 19(2), 155(1978).

33. G. M. Carter, M. K. Thakur, Y. J. Chen and J. V. Hryniewicz, Appl. Phys. Lett., 47,
457(1985).

34. B. I. Greene, J. F. Mueller, J. Orenstein, D. H. Rapkine, S. Schmitt-Rink and M.
Thakur, Phys. Rev. Lett., 61, 325(1988).

35. L. X. Zheng, R. E. Benner, Z. V. Vardeny and G. L. Baker, Physical Review B, 42,
323 5( 1990).

36. S. A. Hambir, D. Wolfe, G. J. Blanchard and G. L. Baker, in preparation.

CHAPTER 5

The Effect of Stretch Orientation on the Linear
and Nonlinear Optical Response of
Poly(4BCMU) Films

Abstract
The effect of stretching-induced alignment on the third order nonlinear optical response of
thin films of the processable polydiacetylene poly(4BCMU) has been investigated. Modest
enhancement of the nonlinear response in the stretch-aligned material compared to the
non-aligned material was observed. The enhancement was limited by the extent to which
the nonlinear polymer and supporting substrate can be stretched. In the stretch-aligned
poly(4BCMU) films, two electronic resonances, centered at 2.02 eV and 2.29 eV, were
detected by using stimulated inverse Raman scattering. The 2.02 eV electronic transition
arises either from crystallinity induced in the spin cast poly(4BCMU) film by alignment or
from an interaction between the substrate and the poly(4BCMU) backbone. The possible

molecular origins for these data are discussed.

93

94
5.1. Introduction

Conjugated polymers have attracted significant attention as candidate materials for
photonic signal processing applications. For a material to be suitable for photonic signal
processing, it must possess a large nonlinear optical response and also be processable.
Many materials satisfy either one of these criteria, but fail to fulfill both. Polydiacetylenes
are one class of polymers that possesses a characteristically large third order (763’)

"'21 and many members of this family of polymers can be

nonlinear optical response,
synthesized as high quality single crystals. The availability of polydiacetylenes in this
structural motif has allowed the unambiguous separation of inter-chain optical effects from

[3“] The ability to effect this separation is

those intrinsic to single polymer chains.
uncommon in most polymeric materials, and the spatial separation of adjacent polymer
backbones by the sidegroups in many crystalline polydiacetylenes has allowed a substantial
level of understanding to be achieved. There have been a large number of nonlinear
optical effects demonstrated in crystalline polydiacetylenes, with several showing promise
for photonic signal processing applications. While crystalline forms of polydiacetylenes
are invaluable for understanding the spectroscopic processes intrinsic to strongly coupled

"‘91 it is unlikely that these materials will be used for practical device

three level systems,
applications because of the difficulties associated with crystal grth as well as the
inability to process these comparatively intractable crystals into usefill device formats.
The production of large-area thin polymer films is a well developed technology‘w’m and is

inexpensive compared to large scale crystal growth. Making thin films of polydiacetylenes

requires that the polymer sidegroups be large and labile enough to allow the polymer to be

95
soluble in organic solvents. Highly uniform thin films of the polydiacetylene

poly(4BCMU) have been demonstrated,[9’“] making this particular polymer a logical
choice for further development. In chapter 4 it was shown that the disorder induced in
poly(4BCMU) as a result of the film formation process served to enhance its inverse
Ramanm] nonlinear switching response per unit length of the polymer.“ The magnitude
of the inverse Raman response in thin films of poly(4BCMU) was the same as that for the
crystalline polymer despite the loss of signal in the film associated with the unavoidable
misalignment between the polymer nonlinear chromophore and the incident electric fields.
The focus of this chapter is on efforts aimed at recovering that portion of the nonlinear
response in films by using stretch alignment techniques. We found that this strategy is
successfill and, in this initial demonstration, is limited by both the elasticity of the
supporting substrate and molecular interactions between the poly(4BCMU) nonlinear

chromophore and the substrate.

5.2. Experimental

5.2.1. Nonlinear laser spectroscopy. A pump-probe laser spectrometer was used for our

[1"] An intense “pump” laser pulse

stimulated inverse Raman scattering measurements.
provides sub-resonant excitation and a weak “probe” laser pulse, approximately one
vibrational resonance higher in energy than the pump pulse, interrogates the result of the
excitation. The spectrometer used for these measurements produces two trains of

picosecond pulses which are independently frequency-tunable and have a well defined

temporal relationship. The source laser is a mode-locked CW Nd:YAG laser that

96
produces 3 W average power at 532 nm and > 1 W average power at 355 nm. The

repetition rate of this laser is ~76 MHz and the pulsewidth is ~100 ps FWHM. The output
of this laser was used to excite synchronously two cavity-dumped dye lasers. The dye
laser pulsewidths were typically 5 ps FWHM and the cross correlation between the two
dye lasers was ~10 ps FWHM. For these experiments the pump laser wavelength was
varied between 579 and 627 nm (Rhodamine 590 and Rhodamine 610, Exciton) and the
probe laser was varied over the range 512 nm to 575 nm (Coumarin 500 and Pyromethene
567, Exciton). The pump-probe signals were detected using radio- and audio-frequency
triple modulation signal encoding with synchronous demodulation detection, allowing shot
noise limited sensitivity and at least 4 orders of magnitude of dynamic rangellsl This wide
dynamic range is necessary for the quantitative measurement of the subtle features present

in inverse Raman data“)

5.2.2. Linear spectroscopy. The absorption data for spin coated and stretch-aligned films
of poly(4BCMU) were recorded with ~1 nm resolution using a Hitachi U4001 absorption

spectrophotometer.

5.2.3. Material processing. Macroscopic chain alignment of spin cast poly(4BCMU)
films was enhanced using stretch-orientation. A 5 wt. % solution of poly(4BCMU) in
cyclopentanone was used for spin coating (1000 rpm, 60 sec.) onto the optically
transparent polymer substrate. After drying, the poly(4BCMU)/substrate composite was

annealed at 100°C for two hours and stretched to the desired elongation ratio (Ella) using

an in-house apparatus.

97
5.2.4. Substrate. The substrate support material used in this work was a copolymer

poly(tetrafluoroethylene-co-hexafluoropropylene) (FEP). This polymer was chosen as the
substrate material because it is amorphous, thereby avoiding Rayleigh scattering
interference to the nonlinear response associated with the processing-induced formation of
microcrystalline domains. Because of the nearly complete fluorination of this polymer, it
possesses a low surface tension and thus does not adhere efficiently to poly(4BCMU).
The surface of the FEP copolymer was modified using a method similar to that used by
Bening and McCarthym] and Dwight and Riggsml The copolymer substrate was treated
with benzophenone and metallic sodium in cyclopentanone at 60°C for 24 hr under N2
atmosphere and was subsequently rinsed with water, acetone and hexane, in that order.
The purpose of this treatment is to abstract fluorine fi’om the FEP copolymer surface and
create carbon-carbon double bonds so that poly(4BCMU) can interact with the polymer
substrate. Spontaneous resonance Raman spectra of the modified substrate exhibited a
resonance at ~1560 cm'1 which was assigned to the C=C stretching mode. The untreated
substrate did not exhibit this resonance. Poly(4BCMU) adheres to the modified FEP

substrate.

98
5.3. Results and Discussion

One of the primary limitations to progress in optical signal processing lies in the ability to
control and process candidate nonlinear materials. To advance this field to the point of
demonstrating practical devices, it will be necessary to effect optical switching in
processable, predominantly non-crystalline materials. While such materials can be made
with comparative ease, understanding their local structure is a substantially more difficult
task. It is this local structure, however, that, to a large extent, determines the nonlinear
optical response(s) of the candidate materials“ The relationship between polymer
morphology and nonlinear optical properties must be understood from a fundamental

perspective to gain predictive control over the processing of these materials.

For practical applications, photonic materials should ideally combine the required optical
properties with favorable mechanical properties, such as rigidity and strength, required for
processability. The most common processing method for poly(4BCMU) is fihn deposition
onto a polymer, glass, or semiconductor substrate by spin coating. The substrate provides
the necessary mechanical characteristics to an otherwise fragile organic polymer. The
thickness of the polymer film can be controlled through the spin coating rate and the
amount of material deposited on the substrate. Because of the comparatively robust
nature of the substrate, it is possible to perform macroscopic stretch alignment operations

on this system in order to optimize the optical response of the photonic material.

99
In previous work on optical waveguiding in spin cast poly(4BCMU) films, high laser

fluence at 1.06 um was shown to change the steady state optical response of the polymer.
This effect is not due to photochemical degradation, but instead to local heating resulting
from the small, but finite linear absorption in poly(4BCMU) at 1.06 um. While optical
switching in poly(4BCMU) is not performed at 1.06 pm, it is necessary to consider
thermally mediated optical processes. To make useful films, they have to be processed in
such a way as to minimize their near-1R absorption tail. The extent of this tail is
determined by local disorder in the polydiacetylene backbone. One important means of
processing poly(4BCMU) is annealing. Differential scanning calorimetry (DSC) traces of
spin-cast poly(4BCMU) films show two broad transitions; one beginning just above room
temperature and extending to 110°C, and a second endothermic transition at 165°C.“81
The first transition is associated with melting of the hydrogen-bonded lattice formed
within the side chains of poly(4BCMU), while the second transition reflects gross
disordering of the polymer backbone. Annealing poly(4BCMU) films at ~100°C sharpens
the first DSC transition and causes a corresponding increase in the intensity of the
excitonic transition. Annealing allows the chains to relax locally and enables reformation
of the hydrogen-bonded lattice. For annealing at T > 110°C, the hydrogen-bonded lattice

“melts” and the chains rearrange to form ordered domains that scatter light.

Heat, solvent, and pressure cause backbone conformational changes in poly(4BCMU) and
these changes are manifested as thenno-, solvato-[w], and piezochromismlzol Particularly
striking are chromic effects that reflect conformational disorder on length scales shorter

than the size of a relaxed exciton on a polydiacteylene chain. For example, blue single

100

crystals of poly(4BCMU) turn red on exposure to solvent vaporsml

and form yellow

[19]

solutions in good solvents. Similarly, heating poly(4BCMU) crystals yields first a red
phase, and then a tacky orange-yellow solid at higher temperatures. In both cases, the red

phase materials are less ordered than the single crystals, and yellow phase materials

correspond to the most extensively disordered polydiacteylene chains.

The linear vibrational and electronic responses of polydiacetylenes provide important
information on the conformation and orientation of the polymer backbone and side
groups. Both the center frequency and the bandwidth of the 7: —) 7t* electronic transition,
localized on the polymer backbone, depend on the processing history of the material and
this relationship can be exploited to characterize and quantitate the extent of backbone
disorder in the polymerml Anisotropy in the electronic response can be used to infer the
extent of backbone ordering in stretch aligned poly(4BCMU). While stretching clearly
improves the overall alignment of the chains, the local environment can remain disordered
because of twists in the polymer backbone, kinks, and side chain disorder. Spectra of
stretch-aligned poly(4BCMU) show a higher optical dichroism than unstretched spin cast
films (vide infra), but the shape of the absorption band for light polarized along the
polymer backbone is largely unchanged from that of unstretched film cast onto the same
substrate (Figure 5.1). A particularly sensitive marker for local disorder in poly(4BCMU)
is the relative intensity of the exciton in polydiacetylenes. In addition, the energy of the
excitonic transition depends sensitively on polymer morphology. For single crystals, the
excitonic resonance appears at ~1.96 eV and dominates the optical properties of

polydiacetylenes. For spin cast poly(4BCMU) films, the exciton appears at ~2.29 eV in

101

1.0 e

0.8 e

Absorbance
O
ON

 

 

b
04— \ /
0.2-
00 1 1 I 1 I 1 14 I 4 I 1 I 1 l
1.6 1.8 2.0 2.2 2.4 2.6 2.8 3.0 3.2
Energy,eV

Figure 5.1. Linear absorption of a poly(4BCMU) film cast on FEP for elongation ratio: (a)
l/t’o = 5 (b) l/Eo = 1.

102
accord with the increased disorder seen for films (Figure 5.2). The excitonic transition

localized on the polymer backbone has been studied extensively in crystalline
polydiacetylenes because it dominates their optical response. In these materials the
exciton is one dimensional to within the width of the backbone 7: system, with a
characteristic length of ~38 Am] The observed exciton binding energy of 400 meV and
very large transition cross-section are direct results of its one-dimensional character and
make this resonance usefill for room temperature nonlinear Optical signal processing
applications. Both the shift and the transition line width can be used to characterize

disorder in these materials.

As discussed above, the linear response of polydiacetylenes is known to depend on
disorder in the polymer backbone. The inverse Raman scattering measurements on
poly(4BCMU) films‘gl spin cast on a quartz substrate demonstrate that the nonlinear
response of this material is also quite sensitive to polymer morphology, and in a manner
that is advantageous for photonic signal processing applications (Figures 5.3-54). While
aging of the spin-cast films shows a significant effect on the linear response of this material
(Figure 5.2), its nonlinear inverse Raman response is largely unaffected, implying long-
term stability of this material for photonic signal processing applications. Inverse Raman
measurements on the crystalline polydiacetylene PTS show that the nonlinear optical
response of polydiacetylenes can be modeled effectively in the context of a strongly
coupled three level system”) For crystalline poly(4BCMU), the inverse Raman
scattering response is virtually identical to that for PTS, while for spin cast films of

poly(4BCMU), the vibronic transition cross sections, resonance energies and linewidths all

103

1.0 -

 

0.8 -

20 i4 20 3'2
Frag/JV

0.6 —

Absorbance
O
A
I

0.2 L-

 

0.0 -

 

I I I

.8 3.0 3.2

I J I I

1 1 1 1 1 I 1
1.6 1.8 2.0 2.2 2.4 2.6 2

Energy, eV

Figure 5.2. Linear absorption of (a) crystalline poly(4BCMU) (b) elongation to l/to = 5
for a poly(4BCMU) film cast on FEP copolymer substrate and (c) fi'eshly
prepared spin cast poly(4BCMU) on a quartz substrate. Inset shows linear
absorption of a four year old spin cast poly(4BCMU) on a quartz substrate.

104

 

 

 

 

0.003 F a) copump = 2.141 eV
0.002
E 0.001 , M
4 " P'eei I.-- ..
0.000 Ljyn’”
-0001
_0.002 5 1 I 1 I 1 I 1 I
2.25 2.30 2.35 2.40 2.45
0) , eV
probe
0003 _ b) mpump = 2.113 6V
0.002
t .
E2 0.001

 

0.000

 

 

 

_0.001 I 1 I 1 I 1 L 1 I L I 1 I
2.28 2.30 2.32 2.34 2.36 2.38 2.40

0) (eV)
probe

Figure 5.3. Experimental data points and calculated fit (solid line) for a) stimulated inverse
Raman spectrum of spin cast poly(4BCMU) for 0),, = 2.141eV and b) for 0),, =
2.113eV.

105

0.003 ..a) co =2.083 eV
_ pump

 

 

 

 

2.20 2.25 2.30 2.35 2.40
0) eV

probe’

b) (1) =1.978 eV

0.005 -

 

 

AT/T

0.000

 

 

-0.00S -

 

I 1 I 1 I 1 I

2.10 2.15 2.20 2.25 2.30

a) , eV
probe

Figure 5.4. Experimental data points and calculated fit (solid line) for a) stimulated inverse

Raman spectrum of spin cast poly(4BCMU) for cop = 2.083eV and b) for top =
1.978eV.

106
differ from those for the crystalline material and depend sensitively on the morphology of

the polymerm

Figures 5.5-5.12 show the stimulated inverse Raman responses of stretch-aligned
poly(4BCMU) films supported on FEP copolymer substrates. These responses, as for the
inverse Raman spectra shown in chapter 4, are characteristic of a system with strong
electronic-vibrational coupling. In the small signal limit, the nonlinear response induced in
the material at a (probe) frequency a). by the action of an intense (pump) electric field at

0),,, the inverse Raman response, can be modeled qualitatively using Equation 1R3]

(3) l/‘Oxlzll‘vxlz 1
1 (cat) = 3 - . 2 . [11
’1 (00x — 601 ‘170x) (00v - wt +60); — 170v)

where the terms it are transition dipole moments and may, coax, 0),, and (or are vibrational,
excitonic and pump (p) and probe (t) laser frequencies, the terms 7 are the linewidths for
the indicated transitions, where O, x and v are ground, excitonic and vibrational energy
levels, respectively."’81 The transition dipole moments and the linewidths both depend on
the morphology of the material. The signal detected experimentally is related to the

change in linear absorption through Equation 2.

— E Aaé [2]

107

00005 I 200) =2.142 eV

pump .. .0‘

0.0004 -

/T
o.
O
O
O
N
IjT‘T

 

 

 

00002"-

2.300 2.325 2.350 2.375 2.400 2.425 2.450
0) eV

probe,

0.0008 r b) 00 = 2.109 eV

 

 

 

 

pump
0.0006 -
0.0004 -
E 0.0002 :- °’
0.0000
0.0002 -
-0.0004 -
1 I 1 I 1 I 1 I 1 I 1 I
2.250 2.275 2.300 2.325 2.350 2.375 2.400
00 ,eV
probe

Figure 5.5. Experimental data (points) and calculated fit (solid line) of the stimulated
inverse Raman spectrum of a spin cast film poly(4BCMU) on a FEP copolymer

substrate, €/€o=l, for (a) 03m= 2.142 eV and (b) 00pm, = 2.109 eV.

108

0.0010- a)w =2.077 ev

0.0008 -'
0.0006 -
E .
< 0.0004 -
0.0002 -

0.0000 ‘\\/
-00002 - W

l ' I

' r ' I ' I ' ' I
2.250 2.275 2.300 2.325 2.350 2.375 2.400

 

 

 

 

 

, eV
probe
rb)c0 =1.9780v .
0.0004 - Pump - -/ \,
" o—o/\"./.\./ \
0.0002
E 0.0000
<

-0.0002

 

 

-00004 1 1 1 1 1 1 1 I
2.150 2.175 2.200 2.225 2.250

(0 eV

probe’

 

Figure 5.6. Experimental data (points) and calculated fit (solid line) of the stimulated
inverse Raman spectrum of a spin cast film poly(4BCMU) on a FEP copolymer

substrate, €/€o=1, for (a) 039m, = 2.077 eV and (b) 03pm= 1.978 eV.

109

a) co = 2.142 eV
pump

F
0.0015 -

0.0010 —

AT/T

0.0005

 

 

0.0000

 

I 1 L L I 1 I 1 I 1 I

2.300 2.325 2.350 2.375 2.400 2.425 2.450
(0 eV

probe,

 

0.0015 r b) 00 = 2.109 eV
pump

0.0010 -

0.0005 *-

T/T

< 0.0000

 

 

-0.0005 -

1 I 1 I 1 I 1 I 1 I 1 J

2.250 2.275 2.300 2.325 2.350 2.375 2.400
00 eV

probe’

 

 

Figure 5.7 . Experimental data (points) and calculated fit (solid line) of the stimulated
inverse Raman spectrum of a spin cast film poly(4BCMU) on a FEP copolymer
substrate, £/€o=l .5, for (a) comp= 2.142 eV and (b) 00pm, = 2.109 eV.

110

0.0012_
0.0010_
0.0008_
0.0006_
0.0004_
0.0002_
0.0000_
-0.0002
-0.0004L.1- 1.1.1... .

2.250 2.275 2.300 2.325 2.350 2.375 2.400

eV

probe,

b) 0 = 1.978 eV
pump

AT/T

 

 

 

 

0.0006 1
0.0004 '
0.0002 '
1.. 0.0000 '
-0.0002 1
-0.0004 1

-0.0006.“*1 -1‘ '
2.150 2.175 2.200 2.225 2.250

0) eV

probe,

 

 

A/T

 

 

Figure 5.8. Experimental data (points) and calculated fit (solid line) of the stimulated
inverse Raman spectrum of a spin cast film poly(4BCMU) on a FEP copolymer

substrate, €/€o=1.5, for (a) com= 2.077 eV and (b) copump= 1.978 eV.

111

a) 0) = 2.142 eV
0.0010 ~ ”mp

0.0008 1
0.0006 '—
0.0004 '-
0.0002 l
0.0000
0.0002

AT/T

 

 

 

 

2.25 2.30 2.35 2.40 2.45
00 cV

probe,

00010 _. b) (0 = 2.109 eV

pump

0.0008 —
0.0006 l
0.0004 3-
% 0.0002 _-
0.0000
00002 i

-OOOO4 , 1 I 1 1 1 J 1 1 1 1 1 I
2.250 2.275 2.300 2.325 2.350 2.375 2.400

00 eV

probe,

 

 

 

 

Figure 5.9. Experimental data (points) and calculated fit (solid line) of the stimulated
inverse Raman spectrum of a spin cast film poly(4BCMU) on a FEP copolymer

substrate, €/£’.,=3, for (a) 00Pump = 2.142 eV and (b) com“, = 2.109 eV.

112

  
 

0. 0008 f a) mpump = 2.077 eV
0.0006 r _.
0.0004 - 2.
E ”0.5". '0
< 0.0002 - '
' l

 

 

 

0.0000
00002 W
1 I 1 I 1 I 1 I

1 I 1 I
2.250 2.275 2.300 2.325 2.350 2.375 2.400

 

 

 

 

0) ,eV
probe
F b)00 =1.978eV
0.0008 - 1111111111 .
0.0006 -
0.0004
5 0.0002
< 0.0000 /'\
-0.0002
'0'0004 1 1 1 1 1 1 E 1 1 1 1 1
2.150 2.175 2.200 2.225 2.250 2.275 2.300
00 ,eV
probe

Figure 5.10. Experimental data (points) and calculated fit (solid line) of the stimulated
inverse Raman spectrum of a spin cast film poly(4BCMU) on a FEP copolymer

substrate, £/£o=3, for (a) (091..., = 2.077 eV and (b) mm= 1.97 8 eV.

113

0.0008 ~
0.0006 "
5 0.0004
< .
0.0002
0.0000
.0.0002

 

 

 

I 1 I 1 I 1 l 1 I 1 I

2.300 2.325 2.350 2.375 2.400 2.425 2.450
(0 eV

probe,

0.0012 _ b) 00 = 2.109 eV

pump
0.0010 f
0.0008 :
0.0006 f -’\,-
0.0004 f
0.0002 :-
0.0000
-0.0002 \/
_0 ' 1 1 L 1 1 1 L 1 1 1 1

04 ‘
2.250 2.275 2.300 2.325 2.350 2.375 2.400
00 6V

probe,

 

AT/T

 

 

 

 

Figure 5.11. Experimental data (points) and calculated fit (solid line) of the stimulated
inverse Raman spectrum of a spin cast film poly(4BCMU) on a FEP copolymer

substrate, M355, for (a) c0Pump = 2.142 eV and (b) (0pm,, = 2.109 eV.

114

a) 10 = 2.077 eV

0.0008
0.0006.
0.0004
0.0002

0.0000 _
-00002 p

-00004 ~ 1 1 4 1 1 1 1 1 1 1 1 1
2.250 2.275 2.300 2.325 2.350 2.375 2.400

00 , eV
probe

AT/T

 

 

 

 

0.0006 _ b) (1) = 1.978 CV

    

0.0004 '
0.0002
E 0.0000 _
00002 -
-00004 —

-0.0006 . 1 ' r
2.150 2.175 2.200

0) , eV
probe

 

 

 

T T

I I
2.225 2.250

Figure 5.12. Experimental data (points) and calculated fit (solid line) of the stimulated
inverse Raman spectrum of a spin cast film poly(4BCMU) on a FEP copolymer

substrate, 020:5, for (a) mm: 2.077 eV and (b) 03m=1978 eV.

1 15
A012 is related to Im{x‘3’} through the dielectric response of the material. By taking the

dielectric response of the polydiacetylenes into account in the calculation of the expected
signal, virtually quantitative agreement between theory and experiment for spin-cast and
stretch-aligned poly(4BCMU) films has been achieved. In all of the inverse Raman
measurements on the poly(4BCMU)/modified FEP system, a positive (AT/T>0)
displacement of the experimental data from the calculated responses has been observed.
This slight difference between the experimental and calculated responses results from two
factors. First, in the development of the theory, Lorentzian lineshape functions were used,
and this is likely not an accurate representation of the experimental lineshapes. The
previous stimulated inverse Raman scattering data on crystalline polydiacetylenes show
that, even for these highly ordered materials, a Lorentzian lineshape does not represent the
actual resonances accurately.[5’7’8] In addition, the uniform background signal observed,
only for the poly(4BCMU) films on polymer substrates, is due either to a coherent
response from the substrate or from an induced spectroscopic feature arising from
interactions between the substrate and the poly(4BCMU). Spontaneous Raman scattering
from the substrate shows a vibrational resonance at 1560 cm'1 superimposed on a broad
background. The background signal in the inverse Raman spectra of the
poly(4BCMU)/FEP system might be of the same origin as that of the broad background

observed in spontaneous Raman scattering data on the modified FEP substrate.

There are two distinct classes of stimulated inverse Raman data that are presented here.
The first set of data is for spin-cast poly(4BCMU) on a quartz substrate annealed at room

temperature for more than four years (Figures 5.3-5.4) and for a freshly prepared

116
poly(4BCMU) spin cast on a quartz substrate (Figure 5.13). These data can be interpreted

in the context of the poly(4BCMU) film not interacting significantly with the substrate,
and only one electronic resonance (2.29 eV) was required to achieve excellent agreement
between experiment and calculation. For these data there is no anomalous offset of the
signal, indicating that there was no detectable response attributable to interactions

between the polymer nonlinear chromophore and the silica substrate.

The second set of data is for poly(4BCMU) spin cast on the modified FEP copolymer
substrate (Figures 5.5-5. 12). For these data, in addition to the constant offset to the signal
discussed above, the agreement between experiment and theory was achieved only when
two excitonic resonances were included in the calculation. In correspondence with the
quartz substrate data, one excitonic resonance was centered at 2.29 eV (7,, 5 50 meV).
The second excitonic resonance, required to achieve agreement with the data, appears at
2.02 eV (7V 5 50 meV). There are two possible reasons for the presence of this additional
electronic resonance, and based on the information at hand, it cannot be unambiguously
determined which explanation is correct. One possibility is that the modification of the
FEP copolymer substrate has introduced sites that interact strongly with the
poly(4BCMU) backbone chromophore. Indeed, the removal of fluorine and introduction
of C=C and CsC bonds onto the substrate surface presents an opportunity for strong TC-‘lt
interaction between the two polymers. Although the characteristics of the modified FEP
surface are not understood completely at the molecular level, the introduction of C=C and
CaC bonds into the FEP substrate produces a material that can be considered to be

composed of, at least in part, methyl-polyacetylene moieties. It is also possible that other

117

0.0003 *- 11

0.0002 -

o/
.

0.0001 -

0.0000 ’ 3 L

-0.0001 F

 

AT/T

 

-0.0002 -

 

 

 

-0.0003 -

0
I 1 I 1 I 1 I

2.10 2.15 2.20 2.25 2.30

(0 , eV
probe

 

 

Figure 5.13. Experimental data (points) and calculated fit (solid line) of the stimulated
inverse Raman spectrum of a freshly prepared spin cast poly(4BCMU) on quartz
substrate for c0pump = 1.97 8 eV including in the calculation single excitonic
resonance (c0x = 2.29 eV).

l 18
types of chemical interaction could be present in this polymer system.

Bening and McCarthy” have shown that oxygen is introduced during rinsing of the FEP
copolymer to form C=O moieties. Blanchard et. al. in their work on the polydiacetylene
PTS have demonstrated the important role that oxygen interactions with the
polydiacetylene backbone can have on the inverse Raman response”) For PTS, an
excitonic resonance at ~2.39 eV (~400 meV higher in energy than the excitonic
resonance) is associated with molecular oxygen adsorbed onto the backbone double bond.
The second electronic resonance observed in poly(4BCMU) film could have a similar
origin, with some oxygen-containing moiety (likely C=O) existing at the interface between
the substrate and the poly(4BCMU), although the energy of the additional resonance
observed in this work is ~270 meV below that of the 2.29 eV exciton. It is also possible,
given the energy of the additional excitonic resonance, that small crystalline domains of
poly(4BCMU) could be re-established in the films as a result of the annealing and
stretching processes. The energy of this additional electronic resonance (2.02 eV) is
within approximately one linewidth of the ~1.96 eV excitonic resonance seen in blue phase
crystalline poly(4BCMU). The transition cross sections for the electronic-to-vibrational
transitions we have measured provide some insight into the origin of this additional
resonance (Table 5.1). The work of Blanchard et. al. on adsorption of oxygen to PTS
showed transition cross sections of o ~1 eV for the adsorbate-induced electronic state-to-
polymer backbone C=C stretch.” For crystalline polydiacetylenes, it was measured that
ocgc E- 0.07 eV and OH; _=_ 0.10 - 0.15 eV,[7’9] depending on the polydiacetylene. For spin

cast films of poly(4BCMU), it was found that ocgc E 0.15 eV and O'c=c E 0.25 eV, with

119
the enhancement compared to the crystalline material being attributable to disorder in the

polymer backbonem For the stretch-aligned films reported here, it was observed that

oc=c _=_ 0.25 eV and ocic 5 0.15 eV

Table 5.1. Cross sections for ground state vibration-to-excited electronic state transitions

in poly(4BCMU).
Crystal Spin-cast on Spin cast on modified FEP
quartz substrate

136/2035

 

(0x = 1.96eV cox = 2.29eV cox = 2.02eV (0x = 2.29eV

 

0H; (eV) 0.15 0.26 0.45 0.25
0C.C (eV) 0.07 0.15 0.25 0.15

for coupling to the 2.29 eV exciton. For the 2.02 eV electronic transition detected in the
stretch-align films, best-fit values of oc=c E 0.45 eV and case E 0.25 eV were obtained.
The cross sections for the 2.02 eV resonance are significantly larger than those for the
crystalline polymer, but are also much less than those observed for the adsorbate-induced
electronic state in PTS. Based on these data, it appears that the 2.02 eV electronic
transition seen in stretch-aligned films arises from an interaction of the poly(4BCMU)
backbone with the substrate, where the interacting substrate moiety was not oxygen. X-
ray diffraction data on these films might aid in determining the origin of this additional

electronic resonance, but such measurements were not feasible on the materials at hand.

It is also possible that the additional excitonic resonance at 2.02 eV was intrinsic to

poly(4BCMU) and its relative contribution to the nonlinear response depends on the

120
annealing history of the polymer. To evaluate this possibility, the inverse Raman

scattering response of both freshly deposited and annealed samples of poly(4BCMU) cast
on quartz substrates were examined. As discussed above, the inverse Raman scattering
response of fresh and four year old poly(4BCMU) samples were identical to within the
uncertainty of the measurements, suggesting the absence of an annealing without
stretching-related origin of the 2.02 eV excitonic resonance seen for the FEP-supported
films. The observed agreement between theory and experiment for calculations with two
excitonic resonances (2.02 eV, 2.29 eV) was poor (Figure 5.14a). The corresponding
calculation, where only the 2.29 eV resonance was included, provides excellent agreement
with the experimental data (Figure 5.14b). For poly(4BCMU) spin cast on a quartz
substrate, only a single excitonic resonance was required to achieve excellent agreement
between theory and experiment. This result demonstrates that the 2.02 eV excitonic
resonance seen for poly(4BCMU) spin cast on the FEP copolymer substrate arises from

the interaction between the two polymers and was not intrinsic to poly(4BCMU).

An underlying assertion in the work presented here is that stretch alignment of
poly(4BCMU) introduces some amount of macroscopic orientation into a material that is,
before processing, isotropic in two dimensions. In order to demonstrate the validity of
this assertion, the linear response of the stretch aligned films parallel and perpendicular to
the stretch alignment axis has been compared. These results are shown in terms of the
anisotropy ratio 001/011 for the supported poly(4BCMU) films stretched to different

elongation ratios (Table 5.2). For the different elongation ratios, 001/011, increases with

elongation, but comparison of these values with the experimental value of 001/011 ~50 for

121

0.0004 -

0.0002 F J
< 0.0000 M A

/T

 

 

 

 

 

 

-0.0002 ~
-0.0004 . 1 . 1 - 1 - 1
2.10 2.15 2.20 2.25 2.30
(0 , eV
probe
0.0004 — b
0.0002 -
t
E-I ,- "1
<1 0.0000 m
1. o 9‘“, .' W
00002 -
~ 2
-0.0004 + 1 . I . 1 . 1
2.10 2.15 2.20 2.25 2.30
(0 , eV
probe

Figure 5.14. Experimental data (points) and calculated fit (solid line) of the stimulated
inverse Raman spectrum of a freshly prepared spin cast poly(4BCMU) on quartz
substrate for c0pump = 1.978 eV including in the calculation (a) two excitonic
resonances (0311 = 2.02, 2.29 eV) (b) single excitonic resonance (cox = 2.29 eV).

122
crystalline poly(4BCMU) demonstrates that the extent of alignment was modest with the

modified FEP substrate. For the elongation ratio, (Mo, spanning the range of 1
(unstretched) to ~5, measured a corresponding variation in 011/011 from 1 to 2.5. To put
these values in perspective, disordering a single crystal with al./011 ~ 50 by exposure to
solvent vapor reduces 0111/01 1 to 5.121] Larger values of (111/011 in the stretch-aligned films

require a significantly larger elongation ratio, although the exact value of {/60 required to

Table 5.2. Comparison of elongation ratio and optical dichroism at 2.29 eV.

 

elongation dichroism
ratio (U130) (011/011)

1 1

1 .5 1 .9

3 2.2

5 2.5

 

achieve all/a1 > 10 cannot be predicted because of the mechanical strain and stress
limitations inherent to both the modified FEP substrate and poly(4BCMU) materials. The
limiting factor to attaining a higher elongation ratio in this work lies with the FEP
copolymer substrate. While other polymers possessing greater deforrnability could be
used, the presence of crystalline domains precludes their use because of significant
Rayleigh scattering losses and interference. These anisotropic linear response experiments
serve to demonstrate that, with the appropriate substrate, a dichroism close to that of a

crystalline material could ultimately be obtained.

123
It is important to understand quantitatively the enhancement in nonlinear response derived

from stretch alignment of these films. Comparison of absolute AT/T quantities was not
meaningfiil because of sample-to-sample variations in the thickness of the poly(4BCMU)
films, and because elongation of the polymer/substrate system serves to thin the

poly(4BCMU) film. As discussed above, the quantity determined experimentally, AT/T ~
A013. By normalizing the data for film thickness (13) and laser intensity (T=I/Io), the

quantity A01 was obtained, which was compared directly for all of the samples (Figure
5.15). There was a significant loss of nonlinear response in the spin-cast, non-aligned film
compared to that of the crystalline polymer, and this was anticipated, at least in part to
misalignment between the polymer backbone and the incident electric fields. Stretch
alignment to an elongation ratio of {/1311 = 5 returns the A01 value to that seen for the
crystalline polymer. It is anticipated that greater elongation of the spin cast films will
allow the realization of a nonlinear response in the disordered material that was
significantly larger than that for the corresponding crystal. Even at this limited elongation
ratio, however, a significant enhancement of the nonlinear response was observed in the
spin cast and stretch aligned system. Because the nonlinear chromophore was
substantially disordered over short (oligomer) length scales, the linear response is
comparatively broad. Accordingly, the nonlinear response of the spin cast and aligned
system exhibits a substantially weaker wavelength-dependence than that seen for
crystalline poly(4BCMU) (Figure 5.15). Thus optical devices based on aligned, spin—cast
films of poly(4BCMU) will exhibit the additional benefit of a comparatively broad

operating frequency range.

124

 

 

140 - —l—- mpump = 2.142eV
. —0— mpump = 2.109eV l
120 " _ __ =
. A (opump 2.077eV .
_ _ = A
100 _ v (opump 1.9786V
80 F
A
a 60 *- v
40 - '/ /
20 1. o '
0 _
1 I 1 I 1 I 1 I 1
1 2 3 4 5

stretch ratio

Figure 5.15. A01, the nonlinear response, normalized for sample thickness and laser
intensities for the various forms of poly(4BCMU) as a function of stretching ratio
for different excitation frequencies.

125
The data presented in Figures 5.5-5. 12 contain information similar to that obtained from

spontaneous resonance Raman scattering measurementsm The polydiacetylene backbone
C=C and CsC stretching resonance frequencies are sensitive to the disorder present in the
stretch aligned films. The positions of both resonances depend on the excitation frequency
(Table 5.3) but the relationship between these two quantities is not direct. The previous

interpretation that the excitation energy-dependence of the vibrational resonances comes

Table 5.3. Excitation energy dependence of vibrational resonance frequencies of spin-cast
and aligned poly(4BCMU) films.

 

 

(”0:1 €/€o=1 .5 €/€o=3 ”30:5
03.0 (CV) C0c=c (Dcac 03C=c (Dc-C (Dc=c (Dcsc 03c=c (Dcac
(CV) (8V) (CV) (CV) (CV) (eV) (CV) (CV)
1.978 0.213 0.245 0.196 0.244 0.182 0.254 0.200 0.249
2.077 0.197 0.265 0.186 0.264 0.187 0.264 0.189 0.265
2.109 0.186 0.260 0.186 0.264 0.191 0.260 0.192 0.265
2.142 0.208 0.263 0.190 0.254 0.178 0.250 0.208 0.254

 

 

 

 

 

about because different excitation frequencies access different regions of the stretch
aligned film that possess a characteristic type or extent of disorder still holds true. Exactly

analogous behavior has been measured in spin cast, unstretched films of poly(4BCMU).

126
5.4. Conclusion

The optical response of poly(4BCMU) thin films supported on a modified FEP copolymer
substrate and processed to different elongation ratios has been investigated. An additional
electronic resonance at 2.02 eV which is due either to crystallinity re-introduced by
stretching and/or annealing or to a strong interaction between the poly(4BCMU)
backbone and the modified FEP substrate has been detected. At this time the origin of this
electronic feature can not be determined unambiguously. While the optical transparency
of the stretch oriented FEP films were of high quality, the elongation ratio attainable for
the substrate used was sufficient only to return the nonlinear response of the orientated
poly(4BCMU) films to that of the crystalline polymer. The nonlinear response of this
composite system could be enhanced by a more suitable choice of polymer substrate to

achieve higher elongation ratios.

127
Literature Cited

1. C. Sauteret, J. P. Hermann, R. Frey, F. Pradere, J. Ducuing, R. H. Baughman and R. R.
Chance, Phys. Rev. Lett., 36, 956(1976).

N

. P. W. Smith, Bell System Tech. Journal, 61, 1975(1982).

b)

. G. J. Blanchard, J. P. Heritage, G. L. Baker and S. Etemad, Chem. Phys. Lett., 158,
329(1989).

4. M. J. Nowak, G. J. Blanchard, G. L. Baker, S. Etemad and Z. G. Soos, Phys. Rev. B,
41, 7933(1990).

5. G. J. Blanchard, J. P. Heritage, Chem. Phys. Lett., 177, 287(1991).

6. S. A. Hambir, T. Yang, G. J. Blanchard and G. L. Baker, Chem. Phys. Lett., 201,
521(1993).

7. G. J. Blanchard, J. P. Heritage, A. C. Von Lehmen, M. K. Kelly, G. L. Baker and S.
Etemad, Phys. Rev. Lett., 63, 887(1989).

8. G. J. Blanchard and J. P. Heritage, J. Chem. Phys., 93, 4377(1990).
9. S. A. Hambir, G. J. Blanchard and G. L. Baker, J. Chem. Phys., 102, 2295(1995).

10. N. E. Schlotter, J. L. Jackel, P. D. Townsend and G. L. Baker, App]. Phys. Lett.,
56,13(1990).

11. P. D. Townsend, J. L. Jackel, G. L. Baker, J. A. Shelbume, III, and S. Etemad, App].
Phys. Lett., 55, 1829(1989).

12. P. D. Townsend, G. L. Baker, NE. Schlotter, C. F. Klausner and S. Etemad, App].
Phys. Lett., 53, 1782(1988).

13. W J. Jones and B. P. Stocheff, Phys. Rev. Lett., 13, 657(1964).
14. Y. Jiang, S. A. Hambir and G. J. Blanchard, Opt. Commun, 99, 216(1993).

15. (a) P. Bado, S. B. Wilson and K. R. Wilson, Rev. Sci. Instrum, 53, 706(1982); (b) L.
Andor, A. Lorincz, J. Siemion, D. D. Smith, and S. A. Rice, Rev. Sci. Instrum., 55,
64(1984); (c) G. J. Blanchard and M. J. Wirth, Anal. Chem, 58, 532(1986).

16. R. C. Bening and T. J. McCarthy, Polym. Prepr., 29, 336(1988).

17. D. W. Dwight and W. J. Riggs, J. Coll. Interface Sci., 47, 650(1974).

128

18. G. L. Baker, J. A. Shelbume, III, and P. D. Townsend, Inst. of Phys. Conf Ser., 103,
227(1989).

19. R. R. Chance, M. W. Washbaugh, and D. J. Hupe, NA T 0 Advanced Research
Workshop; Polydiacetylenes, Synthesis, Structure and Electronic Properties, E102,
39( 1985).

20. B. F. Variano, C. J. Sandroff and G. L. Baker, Macromolecules, 24, 4376(1991).

21. M. J. Nowak, G. J. Blanchard, G. L. Baker, S. Etemad and Z. G. Soos, in Conjugated
Polymeric Materials: Opportunities in Electronic, Opto-Electronics and Molecular
Electronics, Ed. by J. L. Bredas and R. R. Chance, NATO ASI Series E, 182, Kluwer
Academic Publishers, Dordrecht, 421-427(1990).

22. B. I. Greene, J. Orensstein, R. R. Millard, and L. R. Williams, Phys. Rev. Lett., 58,
2750(1987).

23. S. Saikan, N. Hashimoto, T. Kushida and K. Namba, J. Chem. Phys., 82, 5409(1985).

24. L. X. Zheng, R. E. Benner, Z. V. Vardeny and G. L. Baker, Phys. Rev. B, 42,
3235(1990).

Chapter 6

Feasibility Study of Degenerate Four Wave Mixing

This chapter describes an unsuccessful attempt to evaluate the electronic and vibrational
contributions to the third order nonlinear optical response of polydiacetlyenes. We could
not see the degenerate four wave mixing signal due to low laser power and scattering fiom
the sample which results in a small signal to noise ratio. This experiment could be made
successfirl by employing a powerfiil laser, where the peak power is very high, or by

amplification of the laser pulses before they are used in the DFWM process.

129

1 30
6.1. Introduction

Degenerate four wave mixing (DFWM) is a widely used method of measuring 1‘”. The
physical description of DFWM is that three spatially distinguishable electric fields E1(00, t),

E2(c0, t), E3(c0, t) of the same fiequency, 0), interact to generate a fourth wave E4(0), t) at

the same frequency but with a different propagation direction. The equation for the output

154(0), t) will have the following form“’21

 

62E 4 3211
M1 xiii—21
c 5t C d

in which the nonlinear polarization is given by
P = 1 (”13115253 [2]

Many different beam geometries have been used for DFWM. The two common geometries
are forward-wave geometrym (also known as folded boxcars geometry) and the counter
propagating, backward-wave geometrym Figure 6.1 shows the schematic of the forward-
wave geometry. The three input beams labeled 1,2 and 3 are incident on the sample, with
the beam labeled 4 being the output beam, generated by the input beams according to the
susceptibility of the material )6”. One of the incident beams is time delayed relative to the

other beams which are coincident in time. The four beams shown in Figure 1 are emerging

131

10 Q1

30 0.

Figure 6.1. Schematics of the optical beam geometry in the plane of the sample for the
folded boxcars geometry.
from the sample. The generated beam 4 is spatially separated from the intense incident

beams and subsequently detected.

In the backward-wave geometry two beams called the forward (Bf) and backward (E) are
counter propagating and the third beam called the probe (EP) is incident on the sample at
small angle 0 with respect to Ef. For a pulsed laser experiment the pulses are split, each
passing through a separately adjustable delay line, and meet at the sample in a backward-
wave geometry. Beams Ef and Ep are synchronized in time by their cross correlation. Then
the DFWM signal is studied as a function of time delay of the backward beam E. The
DFWM signal obtained from the sample is compared with that obtained from CS; or some
other reference by using the same power level. The )6” value is then obtained by using the

following equation for the case of a nonabsorbing sample“)

132

 

(3) [3]

1 reference 3 sample 1 reference

(3) 0
Z sample _[ "sample ] 6 reference I sample

0
”reference

where nmfmcc and numplc are the linear refractive index of reference and sample,
respectively; 6 firm“ and Z ..mplc are the path lengths of reference and sample, respectively;

Inference and Imp]. are DFWM signal of reference and sample, respectively.
6.2. Experimental

The laser system employed in this experiment is a cavity dumped dye laser pumped by the
second harmonic of Quantronix CW mode locked Nd:YAG laser. To cover the spectrum
below the excitonic resonance in PDAs, the laser dyes DCM, LDS698, and LDS 722 were
used which provide a tuning range from 650 to 750nm. The pulse width of the dye laser is
~5ps. The output of the laser was used as described in the introduction in a folded boxcars
configuration . One of the three beams was chopped at a frequency of about 2KHz. The
signal generated from DFWM would be detected with a photodiode or a photomultiplier

and the output sent to a lock in amplifier.

6.3. Discussion

Many DFWM investigations of nonresonant x6) are based on pumping the ground state to

some virtual state. Pictorially this can be described as in Figure 6.2. If AE can be made

133
equal to the energy difference between the vibrational ground state and the excited state

(Figure 6.2) it is possible to observe coupling between the vibrational and electronic

 

 

 

 

 

 

 

 

X X I A
Virtual -- wr — — ‘4’ Virtual” — 1. — A - — -
v v i "
0 V V o

 

 

 

 

 

 

Figure 6.2. DFWM schematic energy level for the experiments.

resonance. It is expected that both the DFWM and the electronic-vibrational coupling

would contribute to the observed 1‘”. That is

(3) _ (3) 3
I observed _ ZDFWM + Z £311 [41

In this case 193,, would be resonant and would be given by

(3) _ 1‘
Ze-v " .
(op -(a)x —cov)-170v

 

[5]

134

where k at <vlp21x>2

We expect the value of 00,, - ( 0),, «00) z 0, hence even small k would result in a big 19],, .

The transition cross section for the v-x transition can be measured experimentally using

inverse Raman scattering. The nonresonant 1S1);W would be given approximately by

(3) _ k'
2 WM — . [6]
DF 0),, - (ox — 17,,

and Imp - wxl >> 0, hence even if k' is large the denominator is also big, the value of

1827”,, is expected to be small.

6.3.1. Unit conversions: x“) values are reported in the literature in esu, which are
cm4/sC2. In order to calculate an expected E4 for this experiment, we convert x“) to SI
units. We include this information to serve as a USCfUI guide in the conversion between the

two systems.

 

 

l
(3) C722 1.00)?) 45 1 E4 2 g
2.11. = 2 —— 171
327: 208 8 ME1E2E3

13 5
lg): [factor] 1(3)

4 4 2
—CJ =[fac’0 ’61 m2( 1mm) ( 13C 10 j 222,
sC 10000 5.336X10‘ C

 

 

[8]

[factor] = (47:50 )3 [3%) 3

3 — 3
1‘5} - 4 84X10 2" If”),

3 m4 3 C 4m
Table 6.1. Conversion table between 1:92,”, —C—2 and 1591) — .
s

 

184—0'“) 1<I>[——C""]
sC2 S1 J3

 

10'9 4.84 x 10'29

 

10'10 4.84 x 103°

 

 

10'11 4.84 x 10'31

 

 

 

136

6.3.2. Magnitude of the expected signal

The x“) in SI units where all quantities are evaluated in SI units is given by

l

2 2 _
194471802 Cn 210(1),, [451 E4 )2 [10]
3252 202 8 Allfiflbfib

 

 

where c is the speed of light (3 x 108 ms'l); n is the linear refractive index of our sample (n
= 3); 2.0 is the incident wavelength (650nm); (0., is the beam waist at focus (z 20 x 105m);
a is characteristic pulse width ( 0.2 x 101254); I? is the effective interaction length in the
Crystal (z10'7m); M is a factor that corrects for reflection (M z 1); E1, E2 and E3 are the
incident pulse energies (z 1.25 x 10'9 J/pulse, peak power). Substituting these values and

rearranging the above equation to solve for E4 would yield
24 (3) 2
E4 =(2.71X10 1512) [11]

for l.’ = 1000A (10'7m) and different values of 1‘” the average and peak E4 powers are

given in Table 6.2.

137

Table 6.2. Average and peak E values for l = 10'7m and different x”) values.

 

S1

(”[0511] 1’ (m) E«(J/pulse) E4(W)(peak
J3

power)

 

48411105111107 1.72111022 3.441110“

 

4.84x10’30 1x10” 1.721110“ 3.441110"?

 

 

4.84111031 1x10" 1.72111026 3.44111015

 

 

 

 

 

6.3.3. Expected Signal to Noise ratio

Calculation of the signal to noise ratio for x“) = lO'loesu and film thickness 10'7m is shown
below.

Average power = 30 mW

Peak power = 750 W

E = 3.44 x 10'13 W (peak power, signal from DFWM) ( from table 6.2).

Thus, for lsec integration time

Number of signal photons = 5.63 x 1045 photons

Number of noise photons = 1.23 x 1010 photons ( assuming noise is limited by shot noise

on any of the incident lasers).

138

Noise 2 111.23X1010

Signal _ 5.53X10‘6

- = 5.072(10‘ll
Noise J123X1010

The main reason we could not see signal were attributable to the low laser power and very
large scattering. This experiment could be made a success by using a powerful laser or by

amplifying the laser pulse to make the peak power very large.

139

Literature Cited

1. P. N. Prasad, D. J. Williams, Introduction to Nonlinear Optical Effects in Molecules and
Polymers, Wiley Interscience, 1991.

2. Y. R. Shen, Principles of Nonlinear Optics, Wiley Interscience, 1984.

3. J. S. Shirley, R. J. Hall, A. C. Eckbreth, Opt. Lett., 5, 380(1980).

Chapter 7

Suggested Future Studies

This dissertation has discussed the relevance of optical energy storage on the side groups
of polymeric materials to act like an “optical capacitor”. The energy deposited on the side
groups was found to migrate to the polymer backbone in less than 10ps, our instrument
response timeml Even though not successful in storing energy, this work has
demonstrated that the side groups can be used for triggering the optical response of the
polymer backbone. This project can be extended to investigate if the time required for the
energy to migrate from the side group to the polymer backbone can be increased by
increasing the number of CH; spacers between the side group and the polymer backbone.
However, even though the time required for the energy to migrate would increase, this
might be accompanied by the addition of a very large number of structural conformers
when the crystal is formed. Such structural complexity could easily serve to obscure the

desired goal of controlling excitation transport.

One of the main requirements for optical switching is a large third order nonlinearity. The
material of choice was poly(4BCMU) as it can be prepared in wide range of morphologies
varying fiom crystalline to amorphous material. The large )6” requirement for optical
switching can be superseded if an efficient switching scheme and a material that can be

tailored easily to a specific switching application are found. In this dissertation, we have

140

141
shown that inverse Raman scattering (IRS) could be used as an efficient switching scheme

as it is very sensitive to subtle structural changes of materials”) In addition, it was found
that the change in the magnitude of x“) in going from crystalline to spin cast material was
small. This is due to the very large increase in the transition cross section in the spin cast
film. This small decrease in the observed are) is due to the loss of alignment of the
chromophores. The nonlinearity lost to the misalignment of chromophores can be largely
restored by stretch alignment. We have investigated stretch alignment using a polymer
substrate FEP“) Even though a portion of the nonlinearity was restored it is limited by
the stretching ratio attainable with the FEP polymer. This project can be extended to
investigate other polymer substrates with higher stretching ratios the best candidate being
polyethylene terphthalate. To observe a substantial restoration of nonlinearity a stretching

ratio of ~10 will likely be necessary. This project is underway in our laboratory.

Conjugated polymers have received much attention from nonlinear optics community for
potential applications in optical switching. It is the delocalized electrons which are the
basis for the observed nonlinearity. Buckminsterfilllerenes (C60 and C70) are similar
materials containing highly delocalized electrons and so are also interesting candidates in
view of their expected nonlinearity. The large number of papers appearing on the nonlinear
behavior of C60 and C70 show that these materials are attracting attention in the nonlinear
optics communitylmol The future success of x“) nonlinear optical signal processing will,
however, depend on the identification of viable switching strategies in concert with novel
approaches that can use the modest 1(3) nonlinear responses characteristics of organic

materials.

142
Literature Cited

1. S. A. Hambir, T. Yang, G. J. Blanchard and G. L. Baker, Chem. Phys. Lett., 201,
521(1993).

N

. S. A. Hambir, G. J. Blanchard and G. L. Baker, App]. Spec, 49, 374(1995).
3. S. A. Hambir, G. J. Blanchard and G. L. Baker, J. Chem. Phys., 102, 2295(1995).
4. S. A. Hambir, D. Wolfe, G. J. Blanchard and G. L. Baker, in preparation.

5. W. J. Blau, H. J. Byme, D. J. Cardin, T. J. Dennis, J. P. hare, H. W. Droto, R. Taylor,
and D. R. M. Walton, Phys. Rev. Lett., 67, 1423(1991).

6. R. J. Knize and J. P. Partanen, Phys. Rev. Lett., 68, 2704(1992).

7. Z. H. Kafafi, F. J. Bartoli, J. R. Lindle, and R. G. S. Pong, Phys. Rev. Lett., 68,
2705(1992).

8. Z. H. Kafafi, J. R. Lindle, R. G. S. Pong, F. J. Bartoli, L. J. Lingg, and J. Milliken,
Chem. Phys. Lett., 188,492(1992).

9. H. Hoshi, N. Nakamura, Y. Maruyama, T. Nakagawa, S. Suzuki, H. Shiromaru, and Y.
Achiba, Jpn. J. Appl. Phys., 30, L1397(l991).

10. Z. Shang, D. Wang, P. Ye, Y. Li, P. Wu, and D. Zhu, Opt. Lett., 17, 973(1992).

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